When creating a JavaScript date object using a dateString parameter, I expect it to be interpreted in my local time.

For example, since I’m living in Vancouver,

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new Date('January 1, 2012')
//or
new Date('2012/01/01')

returns in Chrome

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"Tue Jan 01 2012 00:00:00 GMT-0800 (PST)".

If I was in Paris, it would return

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"Tue Jan 01 2012 00:00:00 GMT+0100"

etc..

So far so good ! Until I discovered today that a very minor change in my dateString format could make the Date constructor behave unpredictably. Indeed :

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new Date('2012-01-01')

returns in Chrome

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Mon Dec 31 2011 16:00:00 GMT-0800 (PST)

Which is actually Tue Jan 01 2012 00:00:00 but GMT.
My date string was interpreted in universal time instead of local time.

It’s all but trivial since:

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new Date('2012-01-01').getFullYear() //will return 2011 if you're behind GMT !!

ISO 8601 format

The thing is “2012-01-01” is a ISO 8601 format, which support started with javascript 1.8.5.
As “2012/01/01” is a shorthand for “2012/01/01 00:00:00”, “2012-01-01” stands for “2012-01-01T00:00:00”.
Unfortunately, “2012-01-01T00:00:00” implies GMT timezone. If I want to get Jan 1, 2012 in Vancouver time, I have to use “2012-01-01T00:00:00-0800”.

Though it is tempting to use the mysql-like YYYY-MM-DD formatting, it’s probably better to simply avoid it since it’s not supported by old browsers (I tried on IE8 and IE7, it failed !), and tricky when it comes to new ones.

Quick reminder

Here is a quick reminder of the different results I got (you may have other results depending on where you live) :

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/** UTC DATE Reference **/

new Date(Date.UTC(2012,0,1));
new Date('2012');
new Date('2012-01-01');
new Date('2012-01-01T00:00:00+0000');

//=> Mon Dec 31 2011 16:00:00 GMT-0800 (PST)


/** LOCAL DATE */

new Date(2012,0,1);
new Date('January 1, 2012');
new Date('2012/01/01');
new Date('2012-01-01T00:00:00-0800');

//=> Tue Jan 01 2012 00:00:00 GMT-0800 (PST)

Please note:
What is true for Date constructor is also true for Date.parse static method.

Please note 2:
Chrome interprets in local time the following code :

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new Date("2012-01-01 00:00:00");

So the simple fact to add a time makes you jump from GMT to local time.

Firefox doesn’t understand this format and it’s probably for the best.