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When you know what week it is, how to find out the start date of the week

Through the 'date-fns' module, I am receiving numbers of how many weeks the date is this year.
const current = '2022-03-10'
const weekNumber = getWeek(current, 1) // 11
On the contrary, if you only know the numbers, you want to know how to do the first date of the week number.
The way I want to know.
const weekNumber = 11;
const weekOfstartDate = anyFunc(weekNumber) // '2022-03-07'
Do you know the answer to this solution?

You can use the I token:
var dateFns = require("date-fns");
console.log(dateFns.parse('10','I', new Date()));
At npm.runkit.com that returns a date for Mon 7 Mar 2022. It seems date-fns assumes the year of the current week. I have no idea how to specify the year, attempting:
console.log(dateFns.parse('2022-10','YYYY-II', new Date(),{
useAdditionalWeekYearTokens: true
}));
Throws a range error: The format string mustn't contain YYYY and II at the same time. Similarly for "2022W10" and tokens "YYYY'W'II", it says Y and I tokens can't be in the same format string.
A function to do the same thing is:
// Returns the first day (Monday) of the specified week
// Year defaults to the current local calendar year
function getISOWeek(w, y = new Date().getFullYear()) {
let d = new Date(y, 0, 4);
d.setDate(d.getDate() - (d.getDay() || 7) + 1 + 7*(w - 1));
return d;
}
// Mon Mar 14 2022
console.log(getISOWeek(11).toString());
// Mon Jan 02 2023
console.log(getISOWeek(1,2023).toString());
// Mon Dec 28 2026
console.log(getISOWeek(53,2026).toString());
A robust function should validate the input such that the week number is 1 to 53 and that 53 is only permitted for years that have 53 weeks.

I wanted a solution only using date-fns functions. This is how i combined it:
const result = setWeek(nextMonday(new Date(year, 0, 4)), week, {
weekStartsOn: 1,
firstWeekContainsDate: 4,
});

A delivery counter which excludes sunday and holidays

I'm using a delivery Day counter to show my customers on which day they can expect their package.
But... there is a problem.
The delivery days should be (the day ordered + 2 days (before 12 o clock(germany)) if not 3 days)that's working like a charme. But I can't get my head aroung excluding sundays. At the moment the counter adds one day if it is sunday... But for example if it is saturday the counter doesn't add a day (for the nonDeliveryDate)
atm (after 12 o' clock):
17.09.2020 - delivery to Monday = right
18.09.2020 - delivery to Monday = wrong (should be Tuesday)
19.09.2020 - delivery to Tuesday = wrong (should be Wednesday)
20.09.2020 - delivery to Wednesday = right
I think the problem is that the counter only adds one day if "today" is sunday but not if one of the next 3 days is sunday... but i'm not really sure how to solve that.
That's my script:
<script type='text/javascript'>
jQuery(function($) {
// Current date/time
var now = new Date();
// Placeholder for delivery time
var deliveryDate;
// Amount of days to deliver
var deliveryDays = 2;
// Working hours (in UTC -1)
var workingHours = [0 , 9];
// Non-delivery days/dates
// Must match the format returned by .toString():
// Mon Sep 28 1998 14:36:22 GMT-0700 (Pacific Daylight Time)
var nonDelivery = [
"Sun",
"Dec 24",
"Dec 25",
"Dec 31",
"Jan 1"
];
// Create a regular expression
var rxp = new RegExp(nonDelivery.join("|"));
// addDay holds the amount of days to add to delivery date
var addDay = deliveryDays;
// Add an extra day if outside of working hours
var currentHour = now.getUTCHours();
if (currentHour < workingHours[0] ||
currentHour > workingHours[1]) {
addDay++;
}
// Create our delivery date
while (!deliveryDate) {
// Add day(s) to delivery date
now.setDate(
now.getDate() + addDay
);
deliveryDate = now;
if (rxp.test(deliveryDate)) {
addDay = 1;
deliveryDate = false;
}
}
// Format
var locale = "de-DE"; // Our locale
// var day = deliveryDate.toLocaleDateString(locale, { day: "numeric" });
var weekday = deliveryDate.toLocaleDateString(locale, { weekday: "long" });
$('#countdownDate').html( weekday + " " );
});
</script>
Hopefully someone can help me with that...
Thanks a lot!

Javascript, counting years and days since 1970-01-01 until today

I'm trying to count the numbers of years and the days(that remains after the years is counted). So it shows how long its gone in years+days since 1970-01-01. Right now I'm only able to get the years right, and I'm not sure if the days are correct. They are both separated, I need them to in some way make var diffDays and diffYear. A calculation so the computer gets that after counting years, to do minus numbers of years in days and show how many days thats left, since today.
<head>
<script>
var today = new Date();
var dd = today.getDate();
document.write(today);
function myFunction() {
var oneDay = 24*60*60*1000; // hours*minutes*seconds*milliseconds
var firstDate = new Date(1970,01,01);
var secondDate = new Date();
var diffYear = Math.round(Math.abs((firstDate.getTime() - secondDate.getTime())/(oneDay)/365));
var diffDays = Math.round(Math.abs((firstDate.getTime() - secondDate.getTime())/(oneDay)));
document.getElementById("antalYear").innerHTML = diffYear;
document.getElementById("antalDagar").innerHTML = diffDays;
}
</script>
</head>
<body onload="myFunction()">
<p>
Numbers of years and days:
<h3>
<span id="antalYear"></span>
years and
<span id="antalDagar"></span>
days
</h3> Since: 1970,01,01.
</p>
</body>

If you're okay with using a library I would recommend moment.js
It's the go to for handling almost anything related to dates.
var oldDate = moment("1970-01-01", "YYYY-MM-DD")
var today = moment()
console.log(today.diff(oldDate, "years"));
console.log(today.diff(oldDate, "days"));
$(".years").append(today.diff(oldDate, "years"))
$(".days").append(today.diff(oldDate, "days"))
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.23.0/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
Years:<div class="years"></div>
</br>
Days:<div class="days"></div>

For the number of years since 1970, just subtract it from the current year:
date.getFullYear() - 1970
For the number of days since the beginning of the year, you can subtract a date for 1 Jan in the current year from the current date to get milliseconds, then divide by ms per day. However, that doesn't allow for daylight saving changeovers which might affect the result. If you want to round up, so that any time on 1 January is 1 day, etc. then you can set the time to 12 noon, divide by ms/day and round up (or round down and add one). That gives you the day number of the year.
E.g.
function dayOfYear(date) {
// Copy date so don't affect original
var d = new Date(date);
// Get time value for start of 1 Jan in date year
var yearStart = new Date(d.getFullYear(), 0);
// Get number of days, rounded up
return Math.ceil((d.setHours(12,0,0,0) - yearStart) / 8.64e7);
}
// Day of year for current date
console.log('Current day number: ' + dayOfYear(new Date()));
// Day of year for 31 Dec 2016
console.log('Day number for 31 Dec 2016: ' + dayOfYear(new Date(2016, 11, 31))); // 366
If you want the number of completed days (so 1 Jan is 0, 2 Jan is 1, etc.) just subtract 1 from the result (or use Math.floor instead of Math.ceil).

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.

Determining time elapsed between 2 dates

I am trying to determine the time elapsed between 2 dates using javascript. An example would be: "I quit smoking on January 5, 2008 at 3 A.M., how many years, months, and hours has elapsed since I quit?".
So my thoughts were:
Get "quit" date
Get current date
Convert to time (milliseconds)
Find the difference
Create a new date using the difference
Extract the years, months, etc. from that date
Well, it is acting strange and I can't pin point why. Any insight?
//create custom test date
var d1 = new Date(2012, 8, 28, 13, 14, 0, 0);
//create current date
var d2 = new Date();
//get date times (ms)
var d1Time = (d1.getTime());
var d2Time = (d2.getTime());
//calculate the difference in date times
var diff = d2 - d1;
//create a new date using the time differences (starts at Jan 1, 1970)
var dDiff = new Date();
dDiff.setTime(diff);
//chop off 1970 and get year, month, day, and hour
var years = dDiff.getFullYear() - 1970;
var months = dDiff.getMonth();
var days = dDiff.getDate();
var hours = dDiff.getHours();
You can see it in action at this temporary host.

Why don't you just do the math to calculate the values? What you are putting into Date when you do dDiff.setTime(diff); is meaningless to you. That is just going to give you the date diff ms from the epoch.

Changing part of your code may solve your problem. jsfiddle
var start = new Date(0); // pivote point of date.
var years = dDiff.getFullYear() - start.getFullYear();
var months = dDiff.getMonth() - start.getMonth();
var days = dDiff.getDate() - start.getDate();
var hours = dDiff.getHours() - start.getHours();;
console.log(years, months, days, hours);​
But you have to manipulate these values based on there value( they may come negative).

Date represents a particular point in time, not a timespan between two dates.
You are creating a new date by setting dDiff milliseconds ellapsed since the unix epoch.
Once you have the milliseconds ellapsed, you should extract the information you need by dividing it. See this question.
May I recomend taking a look at Moment.js?

This won't be accurate as it does not take into account the leap dayys. Other than that, it is working correctly and I don't see any problem. The time difference is roughly 6.5 days. Taking into account timezone and the fact that 0 is Jan 1st, the value I see is as expected.
The accurate solution would be to
Convert the time difference into days
Subtract the number of leap years elapsed since the specified date
Divide the remaining by 365 to get the number of days
Create an array with the day count of each month (without considering leap days) and loop through the elapsed months, subtracting the day count for the completed months. The number of iterations will be your month count
The remainder is your day count

Various notes:
new Date(2012, 8, 28, 13, 14, 0, 0); is 28 September 2012 13:14:00 (not August if you would it)
new Date(0) returned value is not a constant, because of the practice of using Daylight Saving Time.
dDiff.getMonth(); return 0 for Jan, 1 for Feb etc.
The begin of date (1 Jan 1970) begin with 1 so in difference you should subtract this.
I think the second point is your mistake.
According with your algorithm, try this:
// create date in UTC
//create custom test date
var dlocaltime = new Date(2012, 8, 28, 13, 14, 0, 0);
var d1 = new Date(dlocaltime.getUTCFullYear(),dlocaltime.getUTCMonth(), dlocaltime.getUTCDate(), dlocaltime.getUTCHours(),dlocaltime.getUTCMinutes(),dlocaltime.getUTCSeconds());
//create current date
var now = new Date();
var d2 = new Date(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate(), now.getUTCHours(), now.getUTCMinutes(), now.getUTCSeconds());
console.log(d1);
console.log(d2);
//get date times (ms)
var d1Time = (d1.getTime());
var d2Time = (d2.getTime());
//calculate the difference in date times
var diff = d2 - d1;
//create a new date using the time differences (starts at Jan 1, 1970)
var dDiff = new Date();
dDiff.setTime(diff);
//chop off 1970 and get year, month, day, and hour
var years = dDiff.getUTCFullYear() - 1970;
var months = dDiff.getUTCMonth();
var days = dDiff.getUTCDate()-1; // the date of new Date(0) begin with 1
var hours = dDiff.getUTCHours();
var minutes = dDiff.getUTCMinutes();
var seconds = dDiff.getUTCSeconds();
console.log("Years:"+years);
console.log("months:"+months);
console.log("days:"+days);
console.log("hours:"+hours);
console.log("minutes:"+minutes);
console.log("seconds:"+seconds);