Where does the time go? And when I said where, I meant when?

by Michael S. Kaplan, published on 2007/09/27 03:01 -04:00, original URI: http://blogs.msdn.com/b/michkap/archive/2007/09/27/5110938.aspx

The question I was asked via the Comment link by William was:

Very offtopic since this isn't an internationalization question, but I see you have answered questions about time zones before. So maybe you can answer mine.

Why does the time change happen at 2am?

Well, I could start by making it more of an internationalization question, and point out that it does not change at 2:00 AM for all time zones!

The biggest degree of difference is a time zone that has its daylight savings change happen at noon, and there are several that happen right at midnight or an hour or less from it.

Thankfully neither happens in the US, for very good reasons in both cases:

Assuming that 2 AM is the cutoff everywhere can cause problems as serious as those caused by It doesn't always happen on the hour problems, due to an algorithm making bad assumption on the day or the time....

Although this does not follow the usual definition of a localizability bug, it is not too far off of one given that its primary cause is code assuming that the time zone rules could never be different.

That and the fact that such issues care in a more literal way about remembering that the world is round (the kind of phrase that is used more speculatively to talk about localizability/internationalization issues!), and it ends up feeling entirely relevant and on-topic as a localizability issue! :-)


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# John Cowan on 27 Sep 2007 9:39 AM:

It's also important to notice that some DST transitions happen at a particular moment in local time, whereas others are defined by universal time.  For example, North America makes the DST transition at 2 AM local time in each zone that observes DST, causing the time-zone change to ripple across the continent from Atlantic Time to Aleutian Time.  (Newfoundland/Labrador obey different rules, and Hawaii doesn't observe DST.)

In the EU, on the other hand, all zones transition simultaneously at 1 AM UTC, though the local time varies from one zone to another.  The Olsen time zone database provides full details for the rest of the world.

# Michael S. Kaplan on 27 Sep 2007 11:07 AM:

Hey John, the US situation you describe is the reason for the experience I had in my post The weirdness of the DST change on SIAO.... since this blog is hosted on east coast servers even though I have west coast time zone settings....

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