Calculate Date with start date and number of days with javascript

Question

I am trying to calculate a date from a start date and number of days, so basicly add the number of days to a start date and get an end date. The issue is I get some strange results, but only on one date, I have been a few days now trying to figure this one out.

The function is:

CallculateDateFromDays = function(startDate, days) {
var policy_start_date_array = startDate.split("-");
var policy_start_date = new Date(policy_start_date_array[2], policy_start_date_array[1]-1, policy_start_date_array[0]);
var policy_start_date_add = new Date(policy_start_date.getTime() + ((days-1) * 24 * 60 * 60 * 1000));
var dateString = ("0" + (policy_start_date_add.getDate())).slice(-2) + "-" + ("0" + (policy_start_date_add.getMonth()+1)).slice(-2) + "-" + policy_start_date_add.getFullYear();
return dateString;}

The thing is it works until I use the date "28-10-2012" it gives me back the same date even if I add 2 days.

Any ideas, I am stumped.

Check out [date.js](http://datejs.com/). It has `date.add(n).days()` functionality. — Šime Vidas, Oct 14 '12 at 19:45

score 2 · Answer 1 · answered Oct 14 '12 at 19:40

2

On October 28th the time changes from DST to "normal", so the day is not equal 24h. That may cause issues in your code.

Also why (days-1) * 24 * 60 * 60 * 1000? If you set days to 1 the whole expression evaluates to zero...

answered Oct 14 '12 at 19:40

strah

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because we use this to sell subscriptions by days and if you buy one day only it set the date to the same day – gringoLoco007 Oct 14 '12 at 19:45

Bergi · Accepted Answer · 2012-10-14T19:52:58.180

2

Likely your local timezone changes because of the end of the daylight saving time.

> new Date(2012, 9, 28)
Sun Oct 28 2012 00:00:00 GMT+0200
> // 48 hours later:
> new Date(new Date(2012, 9, 28) + 2 * 24 * 60 * 60 * 1000)
Mon Oct 29 2012 23:00:00 GMT+0100

Always use the UTC methods!

BTW, adding days is much more easier with setDate, which also works around timezone issues:

function calculateDateFromDays(startDate, days) {
    var datestrings = startDate.split("-"),
        date = new Date(+datestrings[2], datestrings[1]-1, +datestrings[0]);
    date.setDate(date.getDate() + days);
    return [("0" + date.getDate()).slice(-2), ("0" + (date.getMonth()+1)).slice(-2), date.getFullYear()].join("-");
}

edited Oct 14 '12 at 19:52

answered Oct 14 '12 at 19:42

Bergi

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Hi, I am putting the string in "14-10-2012" and 1 day and I get back "18-02-2013", what am I doing wrong? – gringoLoco007 Oct 14 '12 at 20:40
Thanks for the tip on UTC, I used it to fix my issue – gringoLoco007 Oct 14 '12 at 21:20

pjam · Answer 3 · 2012-10-14T19:47:58.543

1

There's an easier way to achieve that : http://jsfiddle.net/pjambet/wZEFe/2/

edited Oct 14 '12 at 19:47

answered Oct 14 '12 at 19:40

pjam

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he needs to start with a string, not a date object. – Asad Saeeduddin Oct 14 '12 at 19:46
Try YYYY-mm-dd. The parser only parses that. But the problem is his date is in a different format. – Asad Saeeduddin Oct 14 '12 at 19:48

score 0 · Answer 4 · edited May 23 '17 at 11:43

0

Use javascript date format:

How to format a JavaScript date

So you don't have to manually try to parse date strings.

edited May 23 '17 at 11:43

Community

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answered Oct 14 '12 at 19:39

Mohamed Nuur

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Don't you think that `format` helper function does it manually, too? – Bergi Oct 14 '12 at 19:51

Calculate Date with start date and number of days with javascript

4 Answers4

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