How does javascript know the time zone of a date in milliseconds?

问题

The following code when executed using w3schools' interactive js environment (here):

var d1=new Date(1306796400000);
document.write("Original form: " + d1);

displays the following message:

Original form: Tue May 31 2011 00:00:00 GMT+0100 (GMT Daylight Time)

But this:

var d1=new Date(1231977600000);
document.write("Original form: " + d1);

displays this message:

Original form: Thu Jan 15 2009 00:00:00 GMT+0000 (GMT Standard Time)

I thought that the millisecond value was just milliseconds since 01/01/1970 in UTC. But it looks like it contains a flag for time zone.

Can anyone say what the millisecond value format is?

Thanks in advance.

回答1:

There is no special flag. It's just Daylight Savings in effect.

回答2:

Javascript is executed inside of the user's browser, which in turn, reads the current time zone from user's OS. That's how it can "guess" the proper time zone.

回答3:

The milliseconds do not contain any such flag. However, the time zone of the date object you create using new Date(n) is dependent upon the locale in your interpreter/browser. For me:

var d = new Date(1231977600000);

d.toString();
// "Wed Jan 14 2009 17:00:00 GMT-0700 (Mountain Standard Time)"

d.toUTCString();
// "Thu, 15 Jan 2009 00:00:00 GMT"

回答4:

It does not. It uses local timezone information, including DST transition date. Hence the difference in

javascript:alert([new Date(1306796400000),new Date(1231977600000)].join('\n'))

Set your locale to DST-less timezone and the difference will disappear.

来源：https://stackoverflow.com/questions/4379922/how-does-javascript-know-the-time-zone-of-a-date-in-milliseconds