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 res

Answer 1

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("-");
}

Answer 2

Use javascript date format:

How to format a JavaScript date

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

Answer 3

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...

Answer 4

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