Javascript Date objects and Daylight Savings Time

Question

I'm seeing some behavior I don't understand with Javascript date objects and DST transitions. If I execute the following in Chrome's javascript console

var date = new Date(1268535600000); //2010-03-14T03:00:00.000Z (21:00 03-13 America/Chicago) 
for(var i = 1; i <= 12; i++)
{   
    var time = date.getHours();
    console.log(time)
    console.log(date)
    date.setHours(date.getHours() + 1);         
}

the output is:

21
Sat Mar 13 2010 21:00:00 GMT-0600 (Central Standard Time)
22
Sat Mar 13 2010 22:00:00 GMT-0600 (Central Standard Time)
23
Sat Mar 13 2010 23:00:00 GMT-0600 (Central Standard Time)
0
Sun Mar 14 2010 00:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)

However, changing the last line of the for loop to

date = new Date(date.getTime() + 3600000)   

produces the output I'd expect with the skipped hour at the transition:

21
Sat Mar 13 2010 21:00:00 GMT-0600 (Central Standard Time)
22
Sat Mar 13 2010 22:00:00 GMT-0600 (Central Standard Time)
23
Sat Mar 13 2010 23:00:00 GMT-0600 (Central Standard Time)
0
Sun Mar 14 2010 00:00:00 GMT-0600 (Central Standard Time)
1
Sun Mar 14 2010 01:00:00 GMT-0600 (Central Standard Time)
3
Sun Mar 14 2010 03:00:00 GMT-0500 (Central Daylight Time)
4
Sun Mar 14 2010 04:00:00 GMT-0500 (Central Daylight Time)
5
Sun Mar 14 2010 05:00:00 GMT-0500 (Central Daylight Time)
6
Sun Mar 14 2010 06:00:00 GMT-0500 (Central Daylight Time)
7
Sun Mar 14 2010 07:00:00 GMT-0500 (Central Daylight Time)
8
Sun Mar 14 2010 08:00:00 GMT-0500 (Central Daylight Time)
9
Sun Mar 14 2010 09:00:00 GMT-0500 (Central Daylight Time)

What is the reason the the first approach doesn't work?

Edit: Additionally, with a DST transition with a duplicated hour it seems to just ignore the duplicated hour with the first approach: The other thing is that if I try it with a DST transition with a duplicated hour it just seems to ignore the duplicated hour:

Sun Nov 07 2010 00:00:00 GMT-0500 (Central Daylight Time)
Sun Nov 07 2010 01:00:00 GMT-0600 (Central Standard Time)
Sun Nov 07 2010 02:00:00 GMT-0600 (Central Standard Time)

But it correctly handles the duplicated hour with the second approach.

Answer 1

Maybe it's a bug. Have you tried it on more than one browser?

Otherwise I'd guess that since you're trying to set the hours to a time that doesn't exist, it rejects the change.