Re: dst question

Hi Tom,
thanks for the response.

Tom Lane wrote:
> jgirvin <james.girvin@oasissystems.com.au> writes:
>
>> Australia will come out of DST on the  4th April 2010 at 03:00:00 and
>> will be +9:30 from utc, currently we are +10:30 utc.
>>
>
> OK, so this is a "fall back" transition for you guys, right?
>
Yes this is correct Adelaide, South Aust will be coming out of
(Australian) CDT back into  (Australian) CST.
>
>> This is 4 seconds into the last hour prior to dst changeover, now the
>> to_timestamp result is showing +09:30 as the offset and now() correctly
>> shows +10:30
>> # select now(), to_timestamp( to_char(now(),'yyyy-mm-dd
>> hh24:mi:ss'),'yyyy-mm-dd hh24:mi:ss');
>>
>
>
>>               now                |       to_timestamp
>> ----------------------------------+---------------------------
>>  2010-04-04 02:00:04.841797+10:30 | 2010-04-04 02:00:04+09:30
>> (1 row)
>>
>
>
>> Can someone explain as to why the output from the to_timestamp shows the
>> offset at +09:30 when within the hour of the dst changeover and is this
>> expected....
>>
>
> The problem is that the output of to_char() is ambiguous, since you
> didn't include the timezone in the format spec.  Times between 02:00 and
> 03:00 occur twice on that date, and there's no way to know which time
> "02:00:04" refers to.  The assumption that to_timestamp uses (along with
> our other datetime input code) is that an ambiguous time should be
> resolved as standard time, ie, the second occurrence of "02:00:04".
>
Okay, that makes sense.

>             regards, tom lane
>
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