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How to programmatically determine the prior time period based upon unix time stamps in Javascript?

Essentially I have two unix timestamps, representing the first and last days of a given month. Is it possible programmatically determine the timestamps for the first and last of the previous month?
For example, I have the following two timestamps:
1467331201 --> July 1, 2016
1469923201 --> July 31, 2016
Essentially, can I manipulate these two numbers in a consistent way in order to the unix time (or Date object) for June 1, 2016 and June 30, 2016, respectively? Problem that I'm running into is that you cannot simply subtract a given amount because the amount of days in a month is variable.

You could use this function:
function getPreviousMonthRange(unixTime) {
var dt = new Date(unixTime * 1000);
dt.setUTCDate(0); // flips to the last day of previous month
var unixLast = dt.getTime();
dt.setUTCDate(1); // back to the first day of that same month
var unixFirst = dt.getTime();
return [unixFirst / 1000, unixLast / 1000];
}
// given first and last date (only one is really needed)
var unixTimeFirst = 1467331201;
var unixTimeLast = 1469923201;
// get previous month's first & last date
var [first, last] = getPreviousMonthRange(unixTimeFirst);
// output
console.log('previous month first day: ', first, new Date(first*1000));
console.log('previous month last day: ', last, new Date(last*1000));

Take a look at the following example:
// Specify a timestamp
var timestamp = 1467331201;
// Create a date object for the time stamp, the object works with milliseconds so multiply by 1000
var date = new Date(timestamp * 1000);
// Set the date to the previous month, on the first day
date.setUTCMonth(date.getUTCMonth() - 1, 1);
// Explicitly set the time to 00:00:00
date.setUTCHours(0, 0, 0);
// Get the timestamp for the first day
var beginTimestamp = date.getTime() / 1000;
// Increase the month by one, and set the date to the last day of the previous month
date.setUTCMonth(date.getUTCMonth() + 1, 0);
// Explicitly set the time to 23:59:59
date.setUTCHours(23, 59, 59);
// Get the timestamp for the last day
var endTimestamp = date.getTime() / 1000;
// Print the results
console.log('Timestamps for previous month: ');
console.log('Begin timestamp: ' + beginTimestamp);
console.log('End timestamp: ' + endTimestamp);
A timestamp must be specified in the variable on the top, this might be one of the two timestamps you suggested in your question, anywhere in a month.
This code then calculates the begin and end timestamp for the previous month as you've requested, and prints the results to the console.
Please note, that in this example the begin timestamp uses 00:00:00 as time, and the end timestamp uses 23:59:59 as time (the last second of that day). This can be configured the way you'd prefer.
In this case, we're working with the ...UTC... Date functions, because a Unix timestamp is in UTC time, not in the timezone the user is in.
The statement date.setMonth(date.getMonth() + 1, 0); is used to select the last day in the month. The next month is selected first, but because the day is set to 0 (and not 1) one day is subtracted giving you the preferred result. This is described here.

You can consider using Moment.js. I'm sure this is not exactly how you'd end up handling it but see below for an example of some helpful methods.
var lastDayOfJuly = moment(1469923201);
var firstDayOfJuly = lastDayOfJuly.startOf('month');
var lastDayOfJune = firstDayOfJuly.subtract(1, 'day');
var firstDayOfJune = lastDayOfJune.startOf('month");
Moment.js

Subtract NumOfDays to Given Date to Get PreComputed Date exclude Sundays on Count

I have a problem regarding this
Date : 09/30/2014
NumOfDays : 5
Expected PreComputed Date : 09/24/2014
----------------------------
09/29/2014 Day 1
09/28/2014(Sunday) Skip
09/27/2014 Day 2
09/26/2014 Day 3
09/25/2014 Day 4
09/24/2014 Day 5 ---> PrecComputed Date
I need to get the Precomputed Date Excluding Sundays on Count
Using Javascript or Jquery
on this format mm/dd/yyyy

I know this is spoon feeding but you can iterate the date and check whether there is sunday in any date or not.
function isSunday(date){
return date.getDay()==0;
}
function getPreComputedDate(date1,marginToBeCalc){
var preComputedDate = new Date(date1);
preComputedDate.setHours(0,0,0,0);
for(var i=1;i<=marginToBeCalc-1;i++){
if(isSunday(preComputedDate)){
i--;
}
preComputedDate=addDaysToDate(preComputedDate,1);
}
alert(preComputedDate);
return preComputedDate;
}
function addDaysToDate(date,incrementValue){
var tempDate=new Date(date);
return new Date(tempDate.setDate(tempDate.getDate() + incrementValue));
}
Get Date

Simple javascript date math... not really

I am trying to create a simple script that gives me the next recycling date based on a biweekly schedule starting on Wed Jul 6, 2011. So I've created this simple function...
function getNextDate(startDate) {
if (today <= startDate) {
return startDate;
}
// calculate the day since the start date.
var totalDays = Math.ceil((today.getTime()-startDate.getTime())/(one_day));
// check to see if this day falls on a recycle day
var bumpDays = totalDays%14; // mod 14 -- pickup up every 14 days...
// pickup is today
if (bumpDays == 0) {
return today;
}
// return the closest day which is in 14 days, less the # of days since the last
// pick up..
var ms = today.getTime() + ((14- bumpDays) * one_day);
return new Date(ms);
}
and can call it like...
var today=new Date();
var one_day=1000*60*60*24; // one day in milliseconds
var nextDate = getNextDate(new Date(2011,06,06));
so far so good... but when I project "today" to 10/27/2011, I get Tuesday 11/8/2011 as the next date instead of Wednesday 11/9/2011... In fact every day from now thru 10/26/2011 projects the correct pick-up... and every date from 10/27/2011 thru 2/28/2012 projects the Tuesday and not the Wednesday. And then every date from 2/29/2012 (leap year) thru 10/24/2012 (hmmm October again) projects the Wednesday correctly. What am I missing? Any help would be greatly appreciated..
V

The easiest way to do this is update the Date object using setDate. As the comments for this answer indicate this isn't officially part of the spec, but it is supported on all major browsers.
You should NEVER update a different Date object than the one you did the original getDate call on.
Sample implementation:
var incrementDate = function (date, amount) {
var tmpDate = new Date(date);
tmpDate.setDate(tmpDate.getDate() + amount)
return tmpDate;
};
If you're trying to increment a date, please use this function. It will accept both positive and negative values. It also guarantees that the used date objects isn't changed. This should prevent any error which can occur if you don't expect the update to change the value of the object.
Incorrect usage:
var startDate = new Date('2013-11-01T11:00:00');
var a = new Date();
a.setDate(startDate.getDate() + 14)
This will update the "date" value for startDate with 14 days based on the value of a. Because the value of a is not the same is the previously defined startDate it's possible to get a wrong value.

Expanding on Exellian's answer, if you want to calculate any period in the future (in my case, for the next pay date), you can do a simple loop:
var today = new Date();
var basePayDate = new Date(2012, 9, 23, 0, 0, 0, 0);
while (basePayDate < today) {
basePayDate.setDate(basePayDate.getDate()+14);
}
var nextPayDate = new Date(basePayDate.getTime());
basePayDate.setDate(nextPayDate.getDate()-14);
document.writeln("<p>Previous pay Date: " + basePayDate.toString());
document.writeln("<p>Current Date: " + today.toString());
document.writeln("<p>Next pay Date: " + nextPayDate.toString());
This won't hit odd problems, assuming the core date services work as expected. I have to admit, I didn't test it out to many years into the future...

Note: I had a similar issue; I wanted to create an array of dates on a weekly basis, ie., start date 10/23/2011 and go for 12 weeks. My code was more or less this:
var myDate = new Date(Date.parse(document.eventForm.startDate.value));
var toDate = new Date(myDate);
var week = 60 * 60 * 24 * 7 * 1000;
var milliseconds = toDate.getTime();
dateArray[0] = myDate.format('m/d/Y');
for (var count = 1; count < numberOccurrences; count++) {
milliseconds += week;
toDate.setTime(milliseconds);
dateArray[count] = toDate.format('m/d/Y');
}
Because I didn't specify the time and I live in the US, my default time was midnight, so when I crossed the daylight savings time border, I moved into the previous day. Yuck. I resolved it by setting my time of day to noon before I did my week calculation.

Get a date object (six months prior) from another date object

How can I create a date object which is less than n number of months from another date object? I am looking for something like DateAdd().
Example:
var objCurrentDate = new Date();
Now using objCurrentDate, how can I create a Date object having a date which is six months older than today's date / objCurrentDate?

You can implement very easily an "addMonths" function:
function addMonths(date, months) {
date.setMonth(date.getMonth() + months);
return date;
}
addMonths(new Date(), -6); // six months before now
// Thu Apr 30 2009 01:22:46 GMT-0600
addMonths(new Date(), -12); // a year before now
// Thu Oct 30 2008 01:20:22 GMT-0600
EDIT: As reported by #Brien, there were several problems with the above approach. It wasn't handling correctly the dates where, for example, the original day in the input date is higher than the number of days in the target month.
Another thing I disliked is that the function was mutating the input Date object.
Here's a better implementation handling the edge cases of the end of months and this one doesn't cause any side-effects in the input date supplied:
const getDaysInMonth = (year, month) => new Date(year, month, 0).getDate()
const addMonths = (input, months) => {
const date = new Date(input)
date.setDate(1)
date.setMonth(date.getMonth() + months)
date.setDate(Math.min(input.getDate(), getDaysInMonth(date.getFullYear(), date.getMonth()+1)))
return date
}
console.log(addMonths(new Date('2020-01-31T00:00:00'), -6))
// "2019-07-31T06:00:00.000Z"
console.log(addMonths(new Date('2020-01-31T00:00:00'), 1))
// "2020-02-29T06:00:00.000Z"
console.log(addMonths(new Date('2020-05-31T00:00:00'), -6))
// "2019-11-30T06:00:00.000Z"
console.log(addMonths(new Date('2020-02-29T00:00:00'), -12))
// "2019-02-28T06:00:00.000Z"

Create date object and pass the value of n, where n is number(add/sub) of month.
var dateObj = new Date();
var requiredDate= dateObj.setMonth(dateObj.getMonth() - n);

var oldDate:Date = new Date();
/*
Check and adjust the date -
At the least, make sure that the getDate() returns a
valid date for the calculated month and year.
If it's not valid, change the date as per your needs.
You might want to reset it to 1st day of the month/last day of the month
or change the month and set it to 1st day of next month or whatever.
*/
if(oldDate.getMonth() < n)
oldDate.setFullYear(oldDate.getFullYear() - 1);
oldDate.setMonth((oldDate.getMonth() + n) % 12);

You have to be careful because dates have a lot of edge cases. For example, merely changing the month back by 6 doesn't account for the differing number of days in each month. For example, if you run a function like:
function addMonths(date, months) {
date.setMonth((date.getMonth() + months) % 12);
return date;
}
addMonths(new Date(2020, 7, 31), -6); //months are 0 based so 7 = August
The resulting date to return would be February 31st, 2020. You need to account for differences in the number of days in a month. Other answers have suggested this in various ways, by moving it to the first of the month, or the last of the month, or the first of the next month, etc. Another way to handle it is to keep the date if it is valid, or to move it to the end of the month if it overflows the month's regular dates. You could write this like:
function addMonths(date, months) {
var month = (date.getMonth() + months) % 12;
//create a new Date object that gets the last day of the desired month
var last = new Date(date.getFullYear(), month + 1, 0);
//compare dates and set appropriately
if (date.getDate() <= last.getDate()) {
date.setMonth(month);
}
else {
date.setMonth(month, last.getDate());
}
return date;
}
This at least ensures that the selected day won't "overflow" the month that it is being moved to. Finding the last day of the month with the datePart = 0 method is documented here.
This function still leaves a lot to be desired, as it doesn't add years and you can't subtract more than a year (or you will run into a new issue with negatives being involved). However, fixing those and the other issues you may run into (namely timezones) will be left as an exercise for the reader.

Simple JavaScript not executing?

What could be the cause of the validateDate() function not to execute when called?
The purpose of validateDate() is to take a string like 01/01/2001 and call isValidDate() to determine if the date is valid.
If it's not valid, then a warning message will appear.
function isValidDate(month, day, year){
/*
Purpose: return true if the date is valid, false otherwise
Arguments: day integer representing day of month
month integer representing month of year
year integer representing year
Variables: dteDate - date object
*/
var dteDate;
//set up a Date object based on the day, month and year arguments
//javascript months start at 0 (0-11 instead of 1-12)
dteDate = new Date(year, month, day);
/*
Javascript Dates are a little too forgiving and will change the date to a reasonable guess if it's invalid. We'll use this to our
advantage by creating the date object and then comparing it to the details we put it. If the Date object is different, then it must
have been an invalid date to start with...
*/
return ((day == dteDate.getDate()) && (month == dteDate.getMonth()) && (year == dteDate.getFullYear()));
}
function validateDate(datestring){
month = substr(datestring, 0, 2);
day = substr(datestring, 2, 2);
year = substr(datestring, 6, 4);
if(isValidDate(month, day, year) == false){
alert("Sorry, " + datestring + " is not a valid date.\nPlease correct this.");
return false;
} else {
return true;
}
}

substr is not a function by itself; you must use string.substr(start_index, length).
Since the JavaScript substr method only takes two parameters, not three, this causes execution to halt at the first substr line, and you'll never get output from that function.
I found this by opening Firebug when running your code in a test HTML page. I highly recommend using Firebug for JavaScript debugging.
Try this in your validateDate function, or something similar:
month = datestring.substr(0, 2);
day = datestring.substr(3, 2);
year = datestring.substr(6, 4);

substr is not defined... you need
datestring.substr(0, 2);
you also have a problem with your second substring- it should start at character 3:
day = substr(datestring, 3, 2);
and, month really should be (month - 1) when you create your date

Looking at your code, your date format is "MMDD__YYYY". So your function needs to be as follows:
function isValidDate(month, day, year){
/*
Purpose: return true if the date is valid, false otherwise
Arguments: day integer representing day of month
month integer representing month of year
year integer representing year
Variables: dteDate - date object
*/
var dteDate;
//set up a Date object based on the day, month and year arguments
//javascript months start at 0 (0-11 instead of 1-12)
dteDate = new Date(year, month, day);
alert(d)
/*
Javascript Dates are a little too forgiving and will change the date to a reasonable guess if it's invalid. We'll use this to our
advantage by creating the date object and then comparing it to the details we put it. If the Date object is different, then it must
have been an invalid date to start with...
*/
return ((day == dteDate.getDate()) && (month == dteDate.getMonth()) && (year == dteDate.getFullYear()));
}
function validateDate(datestring){
month = datestring.substring(0, 2);
day = datestring.substring(2, 4);
year = datestring.substring(6, 10);
alert(year)
if(isValidDate(month, day, year) == false){
alert("Sorry, " + datestring + " is not a valid date.\nPlease correct this.");
return false;
} else {
return true;
}
}
validateDate("0202__2010");
If your date is in a more regular format, you can do the following to test if ((new Date("MM/DD/YYYY")) != "Invalid Date")