Question about time period transition points

Paul Koning paul_koning at
Mon Feb 14 19:55:53 UTC 2011

On Feb 14, 2011, at 2:53 PM, Mike Giroux wrote:

> On Mon, Feb 14, 2011 at 2:44 PM, Paul Koning <paul_koning at> wrote:
>> On Feb 14, 2011, at 2:39 PM, Adam Vartanian wrote:
>>>> Phrased differently, for the Americas/NewYork transitions this spring:
>>>> 1) Is the time coordinate value at the instant of transition "2:00 AM" or "3:00
>>>> AM" (NIST says "begins at 2:00 a.m. on the second Sunday of March")
>>>> 2) Is the designation of that coordinate (in the local New York City timezone)
>>>> "EDT" or "EST"?
>>> At least in the United States, time transitions from 1:59:59 AM EST to
>>> 3:00:00 AM EDT.  I don't know if other countries have different rules,
>>> but it would be surprising to me if they did.
>> The changeover time is just as much an arbitrary political decision as all the other aspects of DST change.  So yes,
>> you can find a variety of changeover times.  You can find them in the "AT" column in the rules files.
> Paul,
> That makes sense.  However, combining your answer and Adam's, it looks like the
> "AT" time isn't actually reached?
> The current "northamerica" file shows an AT time of 2:00, not 1:59:59.
> Am I misreading it?

No.  It means at 2:00:00.00000000, so indeed you don't get to see that time because it becomes 3:00:00.000000 DST instead.


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