JavaScript setDate returning wrong dates

Question

I'm getting a date from a string, parsing it to get the day, month and year constituants and use these to instance a Date object.

What I am trying to achieve is to increment the date by one day. It all works fine except that the setDate method insists on returning me invalid dates sometimes...

For example, if I add 1 day to the 28th February 2011, it will return me 29th February 2011... a date which actually doesn't exist.

Is that a bug/limitation of the JavaScript's native Date/Time API, or am I just doing something wrong? I find it hard to believe that it behaves that way without checking the validity of the date.

var myDate = new Date(2011, 2, 28);
console.log(myDate.toString());

myDate.setDate(myDate.getDate() + 1);
console.log(myDate.toString()); // 29 February 2011 !

Thanks.

mplungjan · Accepted Answer · 2011-03-31T09:28:55.497

10

You are not in February - month #2 is MARCH

JS months are 0 based

 var myDate = new Date(2011, 1, 28); // 28th of Feb
 alert(myDate);
 myDate.setDate(myDate.getDate() + 1);
 alert(myDate); // 1st of March 2011 !

PS: Where you MAY have some issues are across the daylight savings time if you are creating dates using var d = new Date() and don't normalise on hours by doing d.setHours(0,0,0,0) afterwards

edited Mar 31 '11 at 09:28

answered Mar 31 '11 at 09:23

mplungjan

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I see now, thanks a lot. I'm coming from a .Net background and the DateTime object does not behave that way in there... I was suspecting I was doing something wrong... can't say I find this behavior very intuitive though, even though most things in programming are 0-based. – Kharlos Dominguez Mar 31 '11 at 09:34
It is even more unintuitive that the year and day parts do not seem to be 0-based... – Kharlos Dominguez Mar 31 '11 at 09:51
@Kharlos - Day numbers are 0 based too - 0 = Sunday - one of those things - more info here: [Why is January month 0 in Java Calendar ?](http://stackoverflow.com/questions/344380/why-is-january-month-0-in-java-calendar) – mplungjan Mar 31 '11 at 09:55
days are 1 based instead. what kind of policy is that? – morels Oct 21 '15 at 09:41
I do not understand your question – mplungjan Oct 21 '15 at 10:03

score 1 · Answer 2 · answered Mar 31 '11 at 09:25

1

No, you are using March, aren't you? 29th of March exists.

var myDate = new Date(2011, 1, 28); // 28th of february

answered Mar 31 '11 at 09:25

Jonas G. Drange

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score 1 · Answer 3 · edited Mar 31 '11 at 09:29

1

You forgot, that it counts months from 0. var myDate = new Date(2011, 2, 28); is actually Mon Mar 28 2011 00:00:00 GMT+0300 (FLE Daylight Time) {}

Try

 var myDate = new Date(2011, 1, 28);
 alert(myDate);
 myDate.setDate(myDate.getDate() + 1);
 alert(myDate); // 1 Mar 2011 !

edited Mar 31 '11 at 09:29

mplungjan

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answered Mar 31 '11 at 09:25

Deele

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JavaScript setDate returning wrong dates

3 Answers3