Validate number of days in a given month - javascript

Performance is of the utmost importance on this one guys... This thing needs to be lightning fast!
How would you validate the number of days in a given month?
My first thought was to make an array containing the days of a given month, with the index representing the month:
var daysInMonth = [
31, // January
28, // February
31, // March
etc.
];
And then do something along the lines of:
function validateDaysInMonth(days, month)
{
if (days < 1 || days > daysInMonth[month]) throw new Error("Frack!");
}
But... What about leap years? How can I implement checking for leap years and keep the function running relatively fast?
Update: I'd like you guys to show me some code which does the days in month- leap year validation.
Here's the flowchart describing the logic used today:
(source: about.com)

function daysInMonth(m, y) { // m is 0 indexed: 0-11
switch (m) {
case 1 :
return (y % 4 == 0 && y % 100) || y % 400 == 0 ? 29 : 28;
case 8 : case 3 : case 5 : case 10 :
return 30;
default :
return 31
}
}
function isValid(d, m, y) {
return m >= 0 && m < 12 && d > 0 && d <= daysInMonth(m, y);
}

I've been doing this using the Date object (assuming it's compiled, and hence blindingly fast compared to scripting).
The trick is that if you enter a too high number for the date part, the Date object wraps over into the next month. So:
var year = 2009;
var month = 1;
var date = 29;
var presumedDate = new Date(year, month, date);
if (presumedDate.getDate() != date)
WScript.Echo("Invalid date");
else
WScript.Echo("Valid date");
This will echo "Invalid date" because presumedDate is actually March 1st.
This leaves all the trouble of leap years etc to the Date object, where I don't have to worry about it.
Neat trick, eh? Dirty, but that's scripting for you...

This will not perform as well as the accepted answer. I threw this in here because I think it is the simplest code. Most people would not need to optimize this function.
function validateDaysInMonth(year, month, day)
{
if (day < 1 || day > 31 || (new Date(year, month, day)).getMonth() != month)
throw new Error("Frack!");
}
It takes advantage of the fact that the javascript Date constructor will perform date arithmetic on dates that are out of range, e.g., if you do:
var year = 2001; //not a leap year!
var month = 1 //February
var day = 29; //not a valid date for this year
new Date(year, month, day);
the object will return Mar 1st, 2001 as the date.

If the month isn't February, get the number from the array. Otherwise, check if the year is leap to return 29, or return 28. Is there a problem with that?

function caldays(m,y)
{
if (m == 01 || m == 03 || m == 05 || m == 07 || m == 08 || m == 10 || m == 12)
{
return 31;
}
else if (m == 04 || m == 06 || m == 09 || m == 11)
{
return 30;
}
else
{
if ((y % 4 == 0) || (y % 400 == 0 && y % 100 != 0))
{
return 29;
}
else
{
return 28;
}
}
}
source: http://www.dotnetspider.com/resources/20979-Javascript-code-get-number-days-perticuler-month-year.aspx

Moment.js
Have you tried moment.js?
The validation is quite easy to use:
var m = moment("2015-11-32");
m.isValid(); // false
I don't know about the performances but hum the project is stared 11,000+ times on GitHub (kind of a quality guarantee).
Source: http://momentjs.com/docs/#/parsing/is-valid/

In computer terms, new Date() and regular expression solutions are slow! If you want a super-fast (and super-cryptic) one-liner, try this one (assuming m is in Jan=1 format):
The only real competition for speed is from #GitaarLab, so I have created a head-to-head JSPerf for us to test on: http://jsperf.com/days-in-month-head-to-head/5
I keep trying different code changes to get the best performance.
Current version
After looking at this related question Leap year check using bitwise operators (amazing speed) and discovering what the 25 & 15 magic number represented, I have come up with this optimized hybrid of answers:
function getDaysInMonth(m, y) {
return m===2 ? y & 3 || !(y%25) && y & 15 ? 28 : 29 : 30 + (m+(m>>3)&1);
}
JSFiddle: http://jsfiddle.net/TrueBlueAussie/H89X3/22/
JSPerf results: http://jsperf.com/days-in-month-head-to-head/5
For some reason, (m+(m>>3)&1) is more efficient than (5546>>m&1) on almost all browsers.
It works based on my leap year answer here: javascript to find leap year this answer here Leap year check using bitwise operators (amazing speed) as well as the following binary logic.
A quick lesson in binary months:
If you interpret the index of the desired months (Jan = 1) in binary you will notice that months with 31 days either have bit 3 clear and bit 0 set, or bit 3 set and bit 0 clear.
Jan = 1 = 0001 : 31 days
Feb = 2 = 0010
Mar = 3 = 0011 : 31 days
Apr = 4 = 0100
May = 5 = 0101 : 31 days
Jun = 6 = 0110
Jul = 7 = 0111 : 31 days
Aug = 8 = 1000 : 31 days
Sep = 9 = 1001
Oct = 10 = 1010 : 31 days
Nov = 11 = 1011
Dec = 12 = 1100 : 31 days
That means you can shift the value 3 places with >> 3, XOR the bits with the original ^ m and see if the result is 1 or 0 in bit position 0 using & 1. Note: It turns out + is slightly faster than XOR (^) and (m >> 3) + m gives the same result in bit 0.
JSPerf results: http://jsperf.com/days-in-month-perf-test/6

I agree with Moayad and TED. Stick with the lookup table unless the month is February. If you need an algorithm for checking leap years, wikipedia has two:
if year modulo 400 is 0 then leap
else if year modulo 100 is 0 then no_leap
else if year modulo 4 is 0 then leap
else no_leap
A more direct algorithm (terms may be grouped either way):
function isLeapYear (year):
if ((year modulo 4 is 0) and (year modulo 100 is not 0)) or (year modulo 400 is 0)
then true
else false

I'm mostly agreeing w/ Moayad. I'd use a table lookup, with an if check on February and the year.
pseudocode:
Last_Day = Last_Day_Of_Month[Month];
Last_Day += (Month == February && Leap_Year(Year)) ? 1 : 0;
Note that Leap_Year() can't be implemented simply as (Year % 4 == 0), because the rules for leap years are way more complex than that. Here's an algorithm cribbed from Wikipedia
bool Leap_Year (int year) {
return ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
}

all this logic is already built in to the javascript engine... Why recode it ? Unless you are doing this just as an exercise, you can use the javascript Date object:
Like this:
function daysInMonth(aDate) {
return new Date(aDate.getYear(), aDate.getMonth()+1, 0).getDate();
}

Assuming the JS Date object standard where months are numbered from 0, and you have your daysInMonth array:
var days = daysInMonth[month] + ((month === 1) && (year % 4 === 0) && ((year % 100 !== 0) || (year % 400 === 0)));
will give you the number of days in the month, with 28 increased to 29 iff the month is February and the year is a leap year.

The days of each month are known easily by exception of February for detail of leap years:
I leave an implementation based on the algorithm that solves this problem:
Algorithm
if (year is not divisible by 4) then (it is a common year)
else if (year is not divisible by 100) then (it is a leap year)
else if (year is not divisible by 400) then (it is a common year)
else (it is a leap year)
More info: https://en.wikipedia.org/wiki/Leap_year#Algorithm
Implementation in Javascript
/**
* Doc: https://en.wikipedia.org/wiki/Leap_year#Algorithm
* param : month is indexed: 1-12
* param: year
**/
function daysInMonth(month, year) {
switch (month) {
case 2 : //Febrary
if (year % 4) {
return 28; //common year
}
if (year % 100) {
return 29; // leap year
}
if (year % 400) {
return 28; //common year
}
return 29; // leap year
case 9 : case 4 : case 6 : case 11 :
return 30;
default :
return 31
}
}
/** Testing daysInMonth Function **/
$('#month').change(function() {
var mVal = parseInt($(this).val());
var yVal = parseInt($('#year').val());
$('#result').text(daysInMonth(mVal, yVal));
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<label>Year</label>
<input type='number' id='year' min='1000' max='2500'>
<label>month</label>
<input type='number' id='month' min='1' max='12'>
<h1>
days: <span id='result' style='color:#E650A0'></span>
</h1>

You can use DateTime to solve this:
new DateTime('20090901')->format('t'); // gives the days of the month

Other solution:
// monthIndex: 0:jan, 11: dic
function daysInMonth (monthIndex, year) {
if (monthIndex ==1) // february case
return (year % 4 == 0 && year % 100) || year % 400 == 0 ? 29 : 28
const monthsWith30days = [3, 5, 8, 10]
return monthsWith30days.includes(monthIndex) ? 30 : 31
}
// testing
const year = new Date().getFullYear()
for (let m=0; m < 12; m += 1) {
console.log(`year ${year} month ${m} has ${daysInMonth(m, year)} days`)
}
OUTPUT
'year 2022 month 0 has 31 days'
'year 2022 month 1 has 28 days'
'year 2022 month 2 has 31 days'
'year 2022 month 3 has 30 days'
'year 2022 month 4 has 31 days'
'year 2022 month 5 has 30 days'
'year 2022 month 6 has 31 days'
'year 2022 month 7 has 31 days'
'year 2022 month 8 has 30 days'
'year 2022 month 9 has 31 days'
'year 2022 month 10 has 30 days'
'year 2022 month 11 has 31 days'

Related

How to use JavaScript to insert days in a month into an option drop down? [duplicate]

I've been using this function but I'd like to know what's the most efficient and accurate way to get it.
function daysInMonth(iMonth, iYear) {
return 32 - new Date(iYear, iMonth, 32).getDate();
}
function daysInMonth (month, year) { // Use 1 for January, 2 for February, etc.
return new Date(year, month, 0).getDate();
}
console.log(daysInMonth(2, 1999)); // February in a non-leap year.
console.log(daysInMonth(2, 2000)); // February in a leap year.
Day 0 is the last day in the previous month. Because the month constructor is 0-based, this works nicely. A bit of a hack, but that's basically what you're doing by subtracting 32.
See more :
Number of days in the current month
Some answers (also on other questions) had leap-year problems or used the Date-object. Although javascript's Date object covers approximately 285616 years (100,000,000 days) on either side of January 1 1970, I was fed up with all kinds of unexpected date inconsistencies across different browsers (most notably year 0 to 99). I was also curious how to calculate it.
So I wrote a simple and above all, small algorithm to calculate the correct (Proleptic Gregorian / Astronomical / ISO 8601:2004 (clause 4.3.2.1), so year 0 exists and is a leap year and negative years are supported) number of day's for a given month and year.
It uses the short-circuit bitmask-modulo leapYear algorithm (slightly modified for js) and common mod-8 month algorithm.
Note that in AD/BC notation, year 0 AD/BC does not exist: instead year 1 BC is the leap-year!
IF you need to account for BC notation then simply subtract one year of the (otherwise positive) year-value first!! (Or subtract the year from 1 for further year-calculations.)
function daysInMonth(m, y){
return m===2?y&3||!(y%25)&&y&15?28:29:30+(m+(m>>3)&1);
}
<!-- example for the snippet -->
<input type="text" value="enter year" onblur="
for( var r='', i=0, y=+this.value
; 12>i++
; r+= 'Month: ' + i + ' has ' + daysInMonth(i, y) + ' days<br>'
);
this.nextSibling.innerHTML=r;
" /><div></div>
Note, months must be 1-based!
Note, this is a different algorithm then the magic number lookup I used in my Javascript calculate the day of the year (1 - 366) answer, because here the extra branch for the leap-year is only needed for February.
If you call this function often, it may be useful to cache the value for better performance.
Here is caching version of FlySwat's answer:
var daysInMonth = (function() {
var cache = {};
return function(month, year) {
var entry = year + '-' + month;
if (cache[entry]) return cache[entry];
return cache[entry] = new Date(year, month, 0).getDate();
}
})();
With moment.js you can use daysInMonth() method:
moment().daysInMonth(); // number of days in the current month
moment("2012-02", "YYYY-MM").daysInMonth() // 29
moment("2012-01", "YYYY-MM").daysInMonth() // 31
To take away confusion I would probably make the month string based as it is currently 0 based.
function daysInMonth(month,year) {
var monthNum = new Date(Date.parse(month +" 1,"+year)).getMonth()+1
return new Date(year, monthNum, 0).getDate();
}
daysInMonth('feb', 2015)
//28
daysInMonth('feb', 2008)
//29
One-liner direct computation (no Date object):
//m is 0-based, Jan = 0, Dec = 11
function daysInMonth(m,y){
return 31-(m-1?m%7&1:y&(y%25?3:15)?3:2);
}
console.log(daysInMonth(1, 2003), "days in February in the non-leap year 2003");
console.log(daysInMonth(1, 2004), "days in February in the leap year 2004");
console.log(daysInMonth(1, 2100), "days in February in the non-leap year 2100");
console.log(daysInMonth(1, 2000), "days in February in the leap year 2000");
console.log(daysInMonth(0, 2022), "days in January 2022");
console.log(daysInMonth(1, 2022), "days in February 2022");
console.log(daysInMonth(2, 2022), "days in March 2022");
console.log(daysInMonth(3, 2022), "days in April 2022");
console.log(daysInMonth(4, 2022), "days in May 2022");
console.log(daysInMonth(5, 2022), "days in June 2022");
console.log(daysInMonth(6, 2022), "days in July 2022");
console.log(daysInMonth(7, 2022), "days in August 2022");
console.log(daysInMonth(8, 2022), "days in September 2022");
console.log(daysInMonth(9, 2022), "days in October 2022");
console.log(daysInMonth(10, 2022), "days in November 2022");
console.log(daysInMonth(11, 2022), "days in December 2022");
Explanation
The main idea is to assume that months have 31 days, but subtract 1 if the month is April, June, September, or November; subtract 2 if the month is February in a leap year; or subtract 3 if the month is February in a non-leap year.
In the ternary expression (m - 1 ? /* Not February */ : /* February */), the expression m - 1 checks whether the month is February.
For other months than February, the expression m % 7 makes m even for months with 31 days, and odd for the rest. Subtracting the lowest bit (& 1) results in 31 − 1 days for April, June, September, and November, and 31 − 0 days for the rest.
For February, the expression y & (y % 25 ? 3 : 15) is falsy for leap years, resulting in 31 − 2 days in February. Otherwise, February is 31 − 3 days.
Here is goes
new Date(2019,2,0).getDate(); //28
new Date(2020,2,0).getDate(); //29
May be bit over kill when compared to selected answer :) But here it is:
function getDayCountOfMonth(year, month) {
if (month === 3 || month === 5 || month === 8 || month === 10) {
return 30;
}
if (month === 1) {
if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
return 29;
} else {
return 28;
}
}
return 31;
};
console.log(getDayCountOfMonth(2020, 1));
I found the above code over here: https://github.com/ElemeFE/element/blob/dev/src/utils/date-util.js
function isLeapYear(year) {
return ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0);
};
const getDaysInMonth = function (year, month) {
return [31, (isLeapYear(year) ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
};
console.log(getDaysInMonth(2020, 1));
I found the above code over here: https://github.com/datejs/Datejs/blob/master/src/core.js
ES6 syntax
const d = (y, m) => new Date(y, m, 0).getDate();
returns
console.log( d(2020, 2) );
// 29
console.log( d(2020, 6) );
// 30
function numberOfDays(iMonth, iYear) {
var myDate = new Date(iYear, iMonth + 1, 1); //find the fist day of next month
var newDate = new Date(myDate - 1); //find the last day
return newDate.getDate(); //return # of days in this month
}
Considering leap years:
function (year, month) {
var isLeapYear = ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0);
return [31, (isLeapYear ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
}
If you want the number of days in the current month of a Date object, consider the following method:
Date.prototype.getNumberOfDaysInMonth = function(monthOffset) {
if (monthOffset !== undefined) {
return new Date(this.getFullYear(), this.getMonth()+monthOffset, 0).getDate();
} else {
return new Date(this.getFullYear(), this.getMonth(), 0).getDate();
}
}
Then you can run it like this:
var myDate = new Date();
myDate.getNumberOfDaysInMonth(); // Returns 28, 29, 30, 31, etc. as necessary
myDate.getNumberOfDaysInMonth(); // BONUS: This also tells you the number of days in past/future months!
In a single line:
// month is 1-12
function getDaysInMonth(year, month){
return month == 2 ? 28 + (year % 4 == 0 ? (year % 100 == 0 ? (year % 400 == 0 ? 1 : 0) : 1):0) : 31 - (month - 1) % 7 % 2;
}
Perhaps not the most elegant solution, but easy to understand and maintain; and, it's battle-tested.
function daysInMonth(month, year) {
var days;
switch (month) {
case 1: // Feb, our problem child
var leapYear = ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
days = leapYear ? 29 : 28;
break;
case 3: case 5: case 8: case 10:
days = 30;
break;
default:
days = 31;
}
return days;
},
If you are going to pass a date variable this may helpful
const getDaysInMonth = date =>
new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();
daysInThisMonth = getDaysInMonth(new Date());
console.log(daysInThisMonth);
One-liner, without using Date objects:
const countDays = (month, year) => 30 + (month === 2 ? (year % 4 === 0 && 1) - 2 : (month + Number(month > 7)) % 2);
returns:
countDays(11,2020) // 30
countDays(2,2020) // 29
countDays(2,2021) // 28
To get the number of days in the current month
var nbOfDaysInCurrentMonth = new Date(Date.UTC(new Date().getUTCFullYear(), new Date().getUTCMonth(), 0)).getDate()
console.log(nbOfDaysInCurrentMonth)
You can get days in month by this command:
new Date(year, month, 0).getDate();
Try this - it returns dictionary with month: days mapping, I think it will be very useful in most cases when people enter this topic:
const getMonthsDaysForYear = (year) => {
let monthDaysDictionary = {};
for(let i = 1; i <= 11; i++) {
const date = new Date(year, i + 1, 0);
const monthName = date.toLocaleString('en-GB', { month: 'long' });
monthDaysDictionary[monthName] = date.getDate();
}
return monthDaysDictionary;
}
getMonthsDaysForYear(2022);
Note: that month should be started with 1 as it is mentioned in this answer.
See my function and a test of it:
function numberOfDays(year, month)
{
// Reference:
// https://arslankuyumculuk.com/how-to-calculate-leap-year-formula/ (2022-05-20 16:45 UTC)
numDays=0;
switch(month)
{
case 1:
numDays=31;
break;
case 2:
numDays=28;
break;
case 3:
numDays=31;
break;
case 4:
numDays=30;
break;
case 5:
numDays=31;
break;
case 6:
numDays=30;
break;
case 7:
numDays=31;
break;
case 8:
numDays=31;
break;
case 9:
numDays=30;
break;
case 10:
numDays=31;
break;
case 11:
numDays=30;
break;
case 12:
numDays=31;
break;
}
if(month==2)
{
if( (year % 100) == 0 )
{
if( (year % 400) == 0 )
{
numDays=29;
}
}
else
{
if( (year % 4) == 0 )
{
numDays=29;
}
}
}
//
return numDays;
}
// Test:
const years = [2019,2020,2021,2022,2023,2024,2025,2026,2027,2028,2100,2400];
month=2;
for (let i = 0; i < years.length; i++)
{
let text = "";
text += years[i] + '/' + month.toString() + ": " + numberOfDays(years[i], month).toString();
alert(text);
}
for (let m = 1; m <= 12; m++)
{
let text2 = "";
text2 += "2022/" + m.toString() + ": " + numberOfDays(2022, m).toString();
alert(text2);
}
#Tomas Langkaas : I've implemented essentially the same function you've mentioned above in awk, with some minor improvements :
using conventional arithmetic instead of bit-wise AND ( & ), plus not using implementation-dependent features, making it fully POSIX-awk-complaint and portable (tested and confirmed working on mawk, gawk, and nawk)
2. user can now directly input actual month numbers of 1 - 12 instead of having to remember to pre-adjust them to 0 - 11
instead of relying on hard-coded numerics, every constant, offset, modulo base etc now dynamically generated on the fly by the function itself ,
while simultaneously being extremely temp variables efficient by recycling both the input month and year variables the moment their original values are no longer required, thus
when February is being called without a year-value, it defaults to NOT-being a leap year, regardless of what the present year is
{m,g,n}awk '
function _____(_,__) {
#
# _| Month mm: [1-12]
# __| Year yyyy:
# |---> #-days:
# in yyyy:mm: combo
return -(\
_^(_<_) != --_ \
? (_ %((_+=_+=_^=_<_) +--_)) %--_\
: (_+=_^=_<_) + (__ == "" ||
__ % (_+=_) ||
__% (_*(_*=_++)+_) == ! (__ % (_*_))\
) ) \
-(_^=_<_) -+- ++_^_^_*_
}'
2 1600 29
2 1700 28
2 1800 28
2 1868 29
2 1900 28
2 1912 29
2 1956 29
2 2000 29
2 2012 29
2 2016 29
2 2018 28
2 2020 29
2 2022 28
2 2024 29

How to reliably implement 'next' / 'previous' month

This has been asked (badly) before - I don't think the answer in that post really addressed the issue, and then it went stale. I'm going to attempt to ask it again with a clearer demonstration of the issue.
The implementation of Javascript Date.setMonth() appears not to follow the principle of least surprise. Try this in a browser console:
d = new Date('2017-08-31') // Set to last day of August
d.getMonth() // 7 - months are zero-based
d.setMonth(8) // Try to set the month to 8 (September)
d.getMonth() // 9 - October. WTF Javascript?
Similarly:
d = new Date('2017-10-31')
d.getMonth() // 9
d.setMonth(8)
d.getMonth() // 9 (still?)
Firefox on Linux appears even worse - sometimes returning a date in October, and a result from getMonth() which doesn't match that month!
My question (and I think that of the OP from that linked question) is how to consistently implement a 'next' / 'prev' month function in, e.g. a datepicker? Is there a well known way of doing this which doesn't surprise the user by, for example, skipping September when they start on August 31st and click 'next'? Going from January 31st is even more unpredictable currently - you will end up on either March 2nd or March 3rd, depending on whether it's a leap year or not!
My personal view is that the least surprise would be to move to the last day of the next / previous month. But that requires the setMonth() implementation to care about the number of days in the months in question, not just add / subtract a fixed duration. According to this thread, the moment.js approach is to add / subtract the number of milliseconds in 30 days, which suggests that library would be prone to the same inconsistencies.
It's all simple and logic. Lets take your example and go see what id does.
So the first line
d = new Date('2017-08-31') // Set to last day of August
console.log(d); // "2017-08-31T00:00:00.000Z"
console.log(d.getMonth()); // 7 - months are zero-based
So all good so far. Next step: Your comment says it: // Try to set the month to 8 (September) So it's not done with trying. You either set it to september or you don't. In your example you set it to October. Explanation further down.
d = new Date('2017-08-31') // Set to last day of August
console.log(d); // "2017-08-31T00:00:00.000Z"
console.log(d.getMonth()); // 7 - months are zero-based
d.setMonth(8) // Try to set the month to 8 (September)
console.log(d); // but now I see I was wrong it is (October)
So the good question is WHY? From MDN
Note: Where Date is called as a constructor with more than one
argument, if values are greater than their logical range (e.g. 13 is
provided as the month value or 70 for the minute value), the adjacent
value will be adjusted. E.g. new Date(2013, 13, 1) is equivalent to
new Date(2014, 1, 1), both create a date for 2014-02-01 (note that the
month is 0-based). Similarly for other values: new Date(2013, 2, 1, 0,
70) is equivalent to new Date(2013, 2, 1, 1, 10) which both create a
date for 2013-03-01T01:10:00.
So that sayd September has only 30 Days but the Date Object has 31. This is why it gives you October and not September.
The simplest will be to take the date you have and set it to first day of month. Something like so:
var d = new Date('2017-08-31') // Set to last day of August
// simplest fix take the date you have and set it to first day of month
d = new Date(d.getFullYear(), d.getMonth(), 1);
console.log(d); // "2017-08-31T00:00:00.000Z"
console.log(d.getMonth()); // 7 - months are zero-based
d.setMonth(8) // Set the month to 8 (September)
console.log(d.getMonth()); // get 8 it is (September)
If setMonth is used when adding and subtracting months, then if the date of the start month doesn't exist in the end month, the extra days cause the date to "roll over" to the next month, so 31 March minus 1 month gives 2 or 3 March.
A simple algorithm is to test the start date and end date and if they differ, set the end date to 0 so it goes to the last day of the previous month.
One issue with this is that subtracting 1 month twice may not give the same result as subtracting 2 months once. 31 March 2017 minus one month gives 28 Feb, minus another month gives 28 Jan. Subtract 2 months from 31 March and you get 31 Jan.
C'est la vie.
function addMonths(date, num) {
var d = date.getDate();
date.setMonth(date.getMonth() + num);
if (date.getDate() != d) date.setDate(0);
return date;
}
// Subtract one month from 31 March
var a = new Date(2017,2,31);
console.log(addMonths(a, -1).toString()); // 28 Feb
// Add one month to 31 January
var b = new Date(2017,0,31);
console.log(addMonths(b, 1).toString()); // 28 Feb
// 29 Feb plus 12 months
var c = new Date(2016,1,29)
console.log(addMonths(c, 12).toString()); // 28 Feb
// 29 Feb minus 12 months
var c = new Date(2016,1,29)
console.log(addMonths(c, -12).toString()); // 28 Feb
// 31 Jul minus 1 month
var d = new Date(2016,6,31)
console.log(addMonths(d, -1).toString()); // 30 Jun
Since getMonth() returns an integer number, you can simply implement a generator over the date object, that sets the month + 1 or - 1 so long as your not at month 11 or month 0 respectively.
function nextMonth(dateObj) {
var month = dateObj.getMonth();
if(month != 11) dateObj.setMonth(month + 1);
return dateObj;
}
function prevMonth(dateObj) {
var month = dateObj.getMonth();
if(month != 0) dateObj.setMonth(month - 1);
return dateObj;
}
If you want to match the days in the previous month you can use an object lookup table.
Now, for your last day of the month problem:
function getLastDayofMonth(month) {
var lookUp = {
0:31,
1:28,
2:30,
3:31
};
return lookUp[month];
}
//and then a revised version
function nextMonth(dateObj) {
var month = dateObj.getMonth();
var day = dateObj.getDate();
if(month != 12) dateObj.setMonth(month + 1);
if(getLastDayofMonth(month)<day)dateObj.setDate(getLastDayofMonth(month));
return dateObj;
}
This should work for incrementing the month, you can use a similar strategy to decrement.
// isLeapYear :: Number -> Boolean
const isLeapYear = ((err) => {
return yr => {
// check for the special years, see https://www.wwu.edu/skywise/leapyear.html
if (yr === 0) {
throw err;
}
// after 8 AD, follows 'normal' leap year rules
let passed = true;
// not technically true as there were 13 LY BCE, but hey.
if (yr === 4 || yr < 0 || (yr % 4)) {
passed = false;
} else {
if (yr % 400) {
if (!(yr % 100)) {
passed = false;
}
}
}
return passed;
};
})(new Error('Year zero does not exist, refers to 1 BCE'));
const daysInMonth = [
31,
28,
31,
30,
31,
30,
31,
31,
30,
31,
30,
31
];
// isLastDay :: Number, Number -> Boolean
const isLastDay = (d, m, y) => {
let dm = isLeapYear(y) && m === 1 ? 29 : daysInMonth(m);
return dm === d;
};
// getLastDay :: Number, Number -> Number
const getLastDay = (m, y) => isLeapYear(y) && m === 1 ? 29 : daysInMonth[m];
// incMonth :: Date -> Date
const incMonth = d => {
let dd = new Date(d.getTime());
let day = dd.getDate();
let month = dd.getMonth() + 1;
dd.setDate(5); // should avoid edge-case shenanigans
dd.setMonth(month);
let year = dd.getFullYear();
if (isLastDay(day, month, year)) day = getLastDay(month, year);
dd.setDate(day);
return dd;
};
This was the solution I came up with, which seems small and reliable as far as I can tell. It doesn't need any extra data structures, and relies on setDate(0) to select the last day of the month in the edge cases. Otherwise it leaves the date alone, which is the behaviour I wanted. It also handles wrapping round from one year to the next (in either direction):
function reallySetMonth(dateObj, targetMonth) {
const newDate = new Date(dateObj.setMonth(targetMonth))
if (newDate.getMonth() !== ((targetMonth % 12) + 12) % 12) { // Get the target month modulo 12 (see https://stackoverflow.com/a/4467559/1454454 for details about modulo in Javascript)
newDate.setDate(0)
}
return newDate
}
Note I've only tested this with targetMonth being either one higher or lower than the current month, since I'm using it with 'next' / 'back' buttons. It would need testing further user with arbitrary months.

javascript to find leap year

How can I get the code below to work when I have a month of february? Currently it is getting to the day and then stopping before getting to the if to determine whether it is a leap year.
if (month == 2) {
if (day == 29) {
if (year % 4 != 0 || year % 100 == 0 && year % 400 != 0) {
field.focus();
field.value = month +'/' + '';
}
}
else if (day > 28) {
field.focus();
field.value = month +'/' + '';
}
}
It's safer to use Date objects for datetime stuff, e.g.
isLeap = new Date(year, 1, 29).getMonth() == 1
Since people keep asking about how exactly this works, it has to do with how JS calculates the date value from year-month-day (details here). Basically, it first calculates the first of the month and then adds N -1 days to it. So when we're asking for the 29th Feb on a non-leap year, the result will be the 1st Feb + 28 days = 1st March:
> new Date(2015, 1, 29)
< Sun Mar 01 2015 00:00:00 GMT+0100 (CET)
On a leap year, the 1st + 28 = 29th Feb:
> new Date(2016, 1, 29)
< Mon Feb 29 2016 00:00:00 GMT+0100 (CET)
In the code above, I set the date to 29th Feb and look if a roll-over took place. If not (the month is still 1, i.e. February), this is a leap year, otherwise a non-leap one.
Compared to using new Date() this is is around 100 times faster!
Update:
This latest version uses a bit test of the bottom 3 bits (is it a multiple of 4), as well as a check for the year being a multiple of 16 (bottom 4 bits in binary is 15) and being a multiple of 25.
ily = function(y) {return !(y & 3 || !(y % 25) && y & 15);};
http://jsperf.com/ily/15
It is slightly faster again than my previous version (below):
ily = function(yr) {return !((yr % 4) || (!(yr % 100) && (yr % 400)));};
http://jsperf.com/ily/7
It is also 5% faster, compared to the already fast conditional operator version by broc.seib
Speed Test results: http://jsperf.com/ily/6
Expected logic test results:
alert(ily(1900)); // false
alert(ily(2000)); // true
alert(ily(2001)); // false
alert(ily(2002)); // false
alert(ily(2003)); // false
alert(ily(2004)); // true
alert(ily(2100)); // false
alert(ily(2400)); // true
isLeap = !(new Date(year, 1, 29).getMonth()-1)
...subtraction by one should work even faster than compare on most CPU architectures.
Correct and Fast:
ily = function(yr) { return (yr%400)?((yr%100)?((yr%4)?false:true):false):true; }
If you are in a loop or counting the nanoseconds, this is two magnitudes faster than running your year through a new Date() object. Compare the performance here: http://jsperf.com/ily
Better historical computation of leap years.
The code below takes into account that leap years were introduced in 45BC with the Julian calendar, and that the majority of the Western world adopted the Gregorian calendar in 1582CE, and that 0CE = 1BC.
isLeap = function(yr) {
if (yr > 1582) return !((yr % 4) || (!(yr % 100) && (yr % 400)));
if (yr >= 0) return !(yr % 4);
if (yr >= -45) return !((yr + 1) % 4);
return false;
};
Britain and its colonies adopted the Gregorian calendar in 1752, so if you are more Anglo centric this version is better (We'll assume Britain adopted the Julian calendar with Roman conquest starting in 43CE).
isLeap = function(yr) {
if (yr > 1752) return !((yr % 4) || (!(yr % 100) && (yr % 400)));
if (yr >= 43) return !(yr % 4);
return false;
};
JavaScript is expected to be getting a new Date/Time API which exposes a new global object - Temporal. This global object provides JS devs with a nicer way to deal with dates/times. It is currently a stage 3 proposal and should hopefully be available for use shortly.
The temporal api exposes a nice property for checking for leap years - inLeapYear. This returns true if a particular date is a leap year, otherwise false. Below we're using with() to convert the date returned by plainDateISO to one with our particular year:
const isLeap = year => Temporal.now.plainDateISO().with({year}).inLeapYear;
console.log(isLeap(2020)); // true
console.log(isLeap(2000)); // true
console.log(isLeap(1944)); // true
console.log(isLeap(2021)); // false
console.log(isLeap(1999)); // false
If you just want to check if your current system date time is a leap year, you can omit the .with():
// true if this year is a leap year, false if it's not a leap year
const isLeap = Temporal.now.plainDateISO().inLeapYear;
I use this because I hate having to keep referring to January as 0 and February as 1.
To me and PHP and readable dates, February=2. I know it doesn't really matter as the number never changes but it just keeps my brain thinking the same across different code.
var year = 2012;
var isLeap = new Date(year,2,1,-1).getDate()==29;
You can easily make this to work calling .isLeapYear() from momentjs:
var notLeapYear = moment('2018-02-29')
console.log(notLeapYear.isLeapYear()); // false
var leapYear = moment('2020-02-29')
console.log(leapYear.isLeapYear()); // true
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.21.0/moment.min.js"></script>
all in one line 😉
const isLeapYear = (year) => (year % 100 === 0 ? year % 400 === 0 : year % 4 === 0);
console.log(isLeapYear(2016)); // true
console.log(isLeapYear(2000)); // true
console.log(isLeapYear(1700)); // false
console.log(isLeapYear(1800)); // false
console.log(isLeapYear(2020)); // true
function isLeap(year) {
if ( (year % 4 === 0 && year % 100 !== 0) || (year % 4 === 0 && year % 100 === 0 && year % 400 === 0) ) {
return 'Leap year.'
} else {
return 'Not leap year.';
}
}
Pseudo code
if year is not divisible by 4 then not leap year
else if year is not divisible by 100 then leap year
else if year is divisible by 400 then leap year
else not leap year
JavaScript
function isLeapYear (year) {
return year % 4 == 0 && ( year % 100 != 0 || year % 400 == 0 )
}
Using the above code insures you do only one check per year if the year is not divisible by 4
Just by adding the brackets you save 2 checks per year that is not divisible by 4
Another alternative is to see if that year has the date of February 29th. If it does have this date, then you know it is a leap year.
ES6
// Months are zero-based integers between 0 and 11, where Febuary = 1
const isLeapYear = year => new Date(year, 1, 29).getDate() === 29;
Tests
> isLeapYear(2016);
< true
> isLeapYear(2019);
< false
function leapYear(year){
if((year%4==0) && (year%100 !==0) || (year%400==0)){
return true;
}
else{
return false;
}
}
var result = leapYear(1700);
console.log(result);
Alternative non-conditionals solution:
const leapYear = y => (y % 4 === 0) + (y % 100 !== 0) + (y % 400 === 0) === 2
Use this:
Date.prototype.isLeap = function() {
return new Date(this.getFullYear(), 1, 29).getMonth() == 1;
};
Date.prototype.isLeap = function() {
return new Date(this.getFullYear(), 1, 29).getMonth() == 1;
};
console.log(new Date("10 Jan 2020").isLeap()); // True
console.log(new Date("10 Jan 2022").isLeap()); // False

Problem with comparing dates in javascript

HI,
I have the following code that is supposed to compare two dates:
var d = ($('#day').val());
var m = ($('#month').val() -1);
var y = $('#year').val();
var birthdate = new Date(y,m,d);
alert('birthdate is' + birthdate);
var today = new Date();
alert('today is'+ today);
var diff = (today - birthdate);
years = Math.floor(diff/(1000*60*60*24*365));
alert(years);
It's basically working but I'm interested to see if the date of birth makes the user over 18 or not. So I've tried to put in 30th march 1993 - which would make the user 17. I'm alerting out the birthdate and it gives me back the correct date (mon mar 29 1993 00:00:00 GMT + 0100 BST)....however this is evaluating to 18 (alert(years) in the above code) when it should evaluate to seventeen. It's not until I put in 3rd April 1993 that it evaluates to 17.
Any ideas?
You have to mind leap-years, timezones... before reinventing the wheel, I recommend that you use DateJS.
if((18).years().ago().isBefore(birthdate)) {
// handle underage visitors
}
That's because you forgot the leap years.
These years had 366 days and occur usually every four years, so in any 18 years there are about four days more than 365*18, thus moving the neccessary start date four days ahead.
Probably in this case it is easier to check
if ((nowyear - birthyear > 18)
|| ((nowyear - birthyear == 18)&&(nowmonth - birthmonth > 0))
|| ((nowyear - birthyear == 18)&&(nowmonth == birthmonth)&&(nowday - birthday >= 0)))
// you're 18!
If you're looking for age, why not just go the simple route and deal with years, months, and days?
function findAge( birthday ){
var today = new Date();
var age = today.getFullYears() - birthday.getFullYears();
if( today.getMonth() - birthday.getMonth() < 0 ){
age--;
}
else if( today.getDay() - birthday.getDay() < 0 && today.getMonth() == birthday.getMonth() ){
age--;
}
}
try to take a look at this post

What is the best way to determine the number of days in a month with JavaScript?

I've been using this function but I'd like to know what's the most efficient and accurate way to get it.
function daysInMonth(iMonth, iYear) {
return 32 - new Date(iYear, iMonth, 32).getDate();
}
function daysInMonth (month, year) { // Use 1 for January, 2 for February, etc.
return new Date(year, month, 0).getDate();
}
console.log(daysInMonth(2, 1999)); // February in a non-leap year.
console.log(daysInMonth(2, 2000)); // February in a leap year.
Day 0 is the last day in the previous month. Because the month constructor is 0-based, this works nicely. A bit of a hack, but that's basically what you're doing by subtracting 32.
See more :
Number of days in the current month
Some answers (also on other questions) had leap-year problems or used the Date-object. Although javascript's Date object covers approximately 285616 years (100,000,000 days) on either side of January 1 1970, I was fed up with all kinds of unexpected date inconsistencies across different browsers (most notably year 0 to 99). I was also curious how to calculate it.
So I wrote a simple and above all, small algorithm to calculate the correct (Proleptic Gregorian / Astronomical / ISO 8601:2004 (clause 4.3.2.1), so year 0 exists and is a leap year and negative years are supported) number of day's for a given month and year.
It uses the short-circuit bitmask-modulo leapYear algorithm (slightly modified for js) and common mod-8 month algorithm.
Note that in AD/BC notation, year 0 AD/BC does not exist: instead year 1 BC is the leap-year!
IF you need to account for BC notation then simply subtract one year of the (otherwise positive) year-value first!! (Or subtract the year from 1 for further year-calculations.)
function daysInMonth(m, y){
return m===2?y&3||!(y%25)&&y&15?28:29:30+(m+(m>>3)&1);
}
<!-- example for the snippet -->
<input type="text" value="enter year" onblur="
for( var r='', i=0, y=+this.value
; 12>i++
; r+= 'Month: ' + i + ' has ' + daysInMonth(i, y) + ' days<br>'
);
this.nextSibling.innerHTML=r;
" /><div></div>
Note, months must be 1-based!
Note, this is a different algorithm then the magic number lookup I used in my Javascript calculate the day of the year (1 - 366) answer, because here the extra branch for the leap-year is only needed for February.
If you call this function often, it may be useful to cache the value for better performance.
Here is caching version of FlySwat's answer:
var daysInMonth = (function() {
var cache = {};
return function(month, year) {
var entry = year + '-' + month;
if (cache[entry]) return cache[entry];
return cache[entry] = new Date(year, month, 0).getDate();
}
})();
With moment.js you can use daysInMonth() method:
moment().daysInMonth(); // number of days in the current month
moment("2012-02", "YYYY-MM").daysInMonth() // 29
moment("2012-01", "YYYY-MM").daysInMonth() // 31
To take away confusion I would probably make the month string based as it is currently 0 based.
function daysInMonth(month,year) {
var monthNum = new Date(Date.parse(month +" 1,"+year)).getMonth()+1
return new Date(year, monthNum, 0).getDate();
}
daysInMonth('feb', 2015)
//28
daysInMonth('feb', 2008)
//29
One-liner direct computation (no Date object):
//m is 0-based, Jan = 0, Dec = 11
function daysInMonth(m,y){
return 31-(m-1?m%7&1:y&(y%25?3:15)?3:2);
}
console.log(daysInMonth(1, 2003), "days in February in the non-leap year 2003");
console.log(daysInMonth(1, 2004), "days in February in the leap year 2004");
console.log(daysInMonth(1, 2100), "days in February in the non-leap year 2100");
console.log(daysInMonth(1, 2000), "days in February in the leap year 2000");
console.log(daysInMonth(0, 2022), "days in January 2022");
console.log(daysInMonth(1, 2022), "days in February 2022");
console.log(daysInMonth(2, 2022), "days in March 2022");
console.log(daysInMonth(3, 2022), "days in April 2022");
console.log(daysInMonth(4, 2022), "days in May 2022");
console.log(daysInMonth(5, 2022), "days in June 2022");
console.log(daysInMonth(6, 2022), "days in July 2022");
console.log(daysInMonth(7, 2022), "days in August 2022");
console.log(daysInMonth(8, 2022), "days in September 2022");
console.log(daysInMonth(9, 2022), "days in October 2022");
console.log(daysInMonth(10, 2022), "days in November 2022");
console.log(daysInMonth(11, 2022), "days in December 2022");
Explanation
The main idea is to assume that months have 31 days, but subtract 1 if the month is April, June, September, or November; subtract 2 if the month is February in a leap year; or subtract 3 if the month is February in a non-leap year.
In the ternary expression (m - 1 ? /* Not February */ : /* February */), the expression m - 1 checks whether the month is February.
For other months than February, the expression m % 7 makes m even for months with 31 days, and odd for the rest. Subtracting the lowest bit (& 1) results in 31 − 1 days for April, June, September, and November, and 31 − 0 days for the rest.
For February, the expression y & (y % 25 ? 3 : 15) is falsy for leap years, resulting in 31 − 2 days in February. Otherwise, February is 31 − 3 days.
Here is goes
new Date(2019,2,0).getDate(); //28
new Date(2020,2,0).getDate(); //29
May be bit over kill when compared to selected answer :) But here it is:
function getDayCountOfMonth(year, month) {
if (month === 3 || month === 5 || month === 8 || month === 10) {
return 30;
}
if (month === 1) {
if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
return 29;
} else {
return 28;
}
}
return 31;
};
console.log(getDayCountOfMonth(2020, 1));
I found the above code over here: https://github.com/ElemeFE/element/blob/dev/src/utils/date-util.js
function isLeapYear(year) {
return ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0);
};
const getDaysInMonth = function (year, month) {
return [31, (isLeapYear(year) ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
};
console.log(getDaysInMonth(2020, 1));
I found the above code over here: https://github.com/datejs/Datejs/blob/master/src/core.js
ES6 syntax
const d = (y, m) => new Date(y, m, 0).getDate();
returns
console.log( d(2020, 2) );
// 29
console.log( d(2020, 6) );
// 30
function numberOfDays(iMonth, iYear) {
var myDate = new Date(iYear, iMonth + 1, 1); //find the fist day of next month
var newDate = new Date(myDate - 1); //find the last day
return newDate.getDate(); //return # of days in this month
}
Considering leap years:
function (year, month) {
var isLeapYear = ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0);
return [31, (isLeapYear ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
}
If you want the number of days in the current month of a Date object, consider the following method:
Date.prototype.getNumberOfDaysInMonth = function(monthOffset) {
if (monthOffset !== undefined) {
return new Date(this.getFullYear(), this.getMonth()+monthOffset, 0).getDate();
} else {
return new Date(this.getFullYear(), this.getMonth(), 0).getDate();
}
}
Then you can run it like this:
var myDate = new Date();
myDate.getNumberOfDaysInMonth(); // Returns 28, 29, 30, 31, etc. as necessary
myDate.getNumberOfDaysInMonth(); // BONUS: This also tells you the number of days in past/future months!
In a single line:
// month is 1-12
function getDaysInMonth(year, month){
return month == 2 ? 28 + (year % 4 == 0 ? (year % 100 == 0 ? (year % 400 == 0 ? 1 : 0) : 1):0) : 31 - (month - 1) % 7 % 2;
}
Perhaps not the most elegant solution, but easy to understand and maintain; and, it's battle-tested.
function daysInMonth(month, year) {
var days;
switch (month) {
case 1: // Feb, our problem child
var leapYear = ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
days = leapYear ? 29 : 28;
break;
case 3: case 5: case 8: case 10:
days = 30;
break;
default:
days = 31;
}
return days;
},
If you are going to pass a date variable this may helpful
const getDaysInMonth = date =>
new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();
daysInThisMonth = getDaysInMonth(new Date());
console.log(daysInThisMonth);
One-liner, without using Date objects:
const countDays = (month, year) => 30 + (month === 2 ? (year % 4 === 0 && 1) - 2 : (month + Number(month > 7)) % 2);
returns:
countDays(11,2020) // 30
countDays(2,2020) // 29
countDays(2,2021) // 28
To get the number of days in the current month
var nbOfDaysInCurrentMonth = new Date(Date.UTC(new Date().getUTCFullYear(), new Date().getUTCMonth(), 0)).getDate()
console.log(nbOfDaysInCurrentMonth)
You can get days in month by this command:
new Date(year, month, 0).getDate();
Try this - it returns dictionary with month: days mapping, I think it will be very useful in most cases when people enter this topic:
const getMonthsDaysForYear = (year) => {
let monthDaysDictionary = {};
for(let i = 1; i <= 11; i++) {
const date = new Date(year, i + 1, 0);
const monthName = date.toLocaleString('en-GB', { month: 'long' });
monthDaysDictionary[monthName] = date.getDate();
}
return monthDaysDictionary;
}
getMonthsDaysForYear(2022);
Note: that month should be started with 1 as it is mentioned in this answer.
See my function and a test of it:
function numberOfDays(year, month)
{
// Reference:
// https://arslankuyumculuk.com/how-to-calculate-leap-year-formula/ (2022-05-20 16:45 UTC)
numDays=0;
switch(month)
{
case 1:
numDays=31;
break;
case 2:
numDays=28;
break;
case 3:
numDays=31;
break;
case 4:
numDays=30;
break;
case 5:
numDays=31;
break;
case 6:
numDays=30;
break;
case 7:
numDays=31;
break;
case 8:
numDays=31;
break;
case 9:
numDays=30;
break;
case 10:
numDays=31;
break;
case 11:
numDays=30;
break;
case 12:
numDays=31;
break;
}
if(month==2)
{
if( (year % 100) == 0 )
{
if( (year % 400) == 0 )
{
numDays=29;
}
}
else
{
if( (year % 4) == 0 )
{
numDays=29;
}
}
}
//
return numDays;
}
// Test:
const years = [2019,2020,2021,2022,2023,2024,2025,2026,2027,2028,2100,2400];
month=2;
for (let i = 0; i < years.length; i++)
{
let text = "";
text += years[i] + '/' + month.toString() + ": " + numberOfDays(years[i], month).toString();
alert(text);
}
for (let m = 1; m <= 12; m++)
{
let text2 = "";
text2 += "2022/" + m.toString() + ": " + numberOfDays(2022, m).toString();
alert(text2);
}
#Tomas Langkaas : I've implemented essentially the same function you've mentioned above in awk, with some minor improvements :
using conventional arithmetic instead of bit-wise AND ( & ), plus not using implementation-dependent features, making it fully POSIX-awk-complaint and portable (tested and confirmed working on mawk, gawk, and nawk)
2. user can now directly input actual month numbers of 1 - 12 instead of having to remember to pre-adjust them to 0 - 11
instead of relying on hard-coded numerics, every constant, offset, modulo base etc now dynamically generated on the fly by the function itself ,
while simultaneously being extremely temp variables efficient by recycling both the input month and year variables the moment their original values are no longer required, thus
when February is being called without a year-value, it defaults to NOT-being a leap year, regardless of what the present year is
{m,g,n}awk '
function _____(_,__) {
#
# _| Month mm: [1-12]
# __| Year yyyy:
# |---> #-days:
# in yyyy:mm: combo
return -(\
_^(_<_) != --_ \
? (_ %((_+=_+=_^=_<_) +--_)) %--_\
: (_+=_^=_<_) + (__ == "" ||
__ % (_+=_) ||
__% (_*(_*=_++)+_) == ! (__ % (_*_))\
) ) \
-(_^=_<_) -+- ++_^_^_*_
}'
2 1600 29
2 1700 28
2 1800 28
2 1868 29
2 1900 28
2 1912 29
2 1956 29
2 2000 29
2 2012 29
2 2016 29
2 2018 28
2 2020 29
2 2022 28
2 2024 29

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