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POJavascript Date objects and Daylight Savings Time
 

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    <p>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</p> <pre><code>var date = new Date(1268535600000); //2010-03-14T03:00:00.000Z (21:00 03-13 America/Chicago) for(var i = 1; i &lt;= 12; i++) { var time = date.getHours(); console.log(time) console.log(date) date.setHours(date.getHours() + 1); } </code></pre> <p>the output is:</p> <pre><code>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) </code></pre> <p>However, changing the last line of the for loop to</p> <pre><code>date = new Date(date.getTime() + 3600000) </code></pre> <p>produces the output I'd expect with the skipped hour at the transition:</p> <pre><code>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) </code></pre> <p>What is the reason the the first approach doesn't work?</p> <p>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:</p> <pre><code>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) </code></pre> <p>But it correctly handles the duplicated hour with the second approach.</p>
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