time during standard to DST transition

Scott Atwood scott.roy.atwood at gmail.com
Thu Dec 11 19:15:12 UTC 2008

On Thu, Dec 11, 2008 at 10:59 AM, Paul Koning <Paul_Koning at dell.com> wrote:

> >>>>> "Jennifer" == Jennifer Wang <(jennifwa)" <jennifwa at cisco.com>>
> writes:
>  Jennifer> I think in most case when a caller sets the time to 2:10am
>  Jennifer> Mar 8, 2009 in America/Los_Angeles, he does not realize
>  Jennifer> it's the "missing hour".  If he wants 1:10am (which is a
>  Jennifer> valid time), he would use that.  So moving forward to
>  Jennifer> 3:10am seems to make sense.
> No, it doesn't.  If he meant 3:10 he would have said so.  There is no
> basis at all to guess at the user's intent in this way.
> It's very simple.  The user is asking for a non-existent time.  So the
> reject is valid.  It is every bit as valid as an attempt to set the
> current date/time to February 30th.

That's actually a poorly chosen example, since mktime() does guess the users
intent in a case like that, by normalizing fields that are out of range.
February 30th would be normalized to March 1st or March 2nd (depending on
the year).

However, I still contend that these cases are different.  In the case of a
field that goes beyond its normal range, you can clearly and unambiguously
determine the user's intent, since mktime() was designed to make it easy to
do something like calculate a date that is, say, 14 days in the future or
past by simply adding or subtracting fourteen days on the tm_mday field.
Then mktime() takes the denormalized representation and does the right

In the case of the non-existent hour during a DST transition, there is an
inherent ambiguity, since you don't know if the caller added to or
subtracted from the tm_hour field to get to the invalid time.


Scott Atwood

Cycle tracks will abound in Utopia.  ~H.G. Wells
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