Re: timestamp with timezone and time zone name

Steve Crawford <scrawford@xxxxxxxxxxxxxxxxxxxx> · Wed, 08 Aug 2012 10:13:41 -0700



    On 08/07/2012 08:36 PM, Shridhar
      Daithankar wrote:

    
      On Tuesday 07 Aug 2012 12:21:04 AM Tom Lane
        wrote:
      > Shridhar Daithankar
        <ghodechhap@xxxxxxxxxxxxxx> writes:
      > > I am wondering, why following two
        values result in a shift by 3.5 hours. I
      > > would expect them to be identical.
      > > 
      > > I understand that canonical time
        zone names could be ambiguous at times
      > > but I think IST is not one of them.
      > 
      > I don't know why you'd think that ...
      > 
      > src/timezone/tznames/Asia.txt:IST 19800
        # Indian Standard Time
      > src/timezone/tznames/Asia.txt:IST 7200 #
        Israel Standard Time
       
      My bad.. should have searched a bit more.
      > 
      > ... and there's some references to
        "Irish Summer Time" in the Olson
      > database, as well. IIRC, IST was one of
        the primary problems that
      > forced us to invent the
        "timezone_abbreviations" configuration
      > mechanism. Try setting that to "India"
        if you want the 05:30 meaning.
       
      Thanks. I will stick to the numerical offsets
        for uniformity.
    
    
    Date/time processing has lots of potential gotchas. Spend some
    quality-time with:

    http://www.postgresql.org/docs/current/static/datatype-datetime.html

    
    Note that short abbreviations and numerical offsets are simple
    offsets from UTC and will require you to change the offset as
    appropriate for each timestamp you want to enter. For example it is
    perfectly OK to enter midnight on new-year's day Eastern Daylight
    Time. PostgreSQL sees this as an offset of 4-hours from UTC which
    you could also specify as -04. Since January 1 is winter in New
    York, this may not be what you want.

    
    select '2012-01-01 0000 EDT'::timestamptz at time zone 'UTC';

          timezone       

    ---------------------

     2012-01-01 04:00:00

    
    If you want PostgreSQL to account for DST rules (including how the
    rules have changed historically), use the timezone name:

    
    select '2012-01-01 0000 America/New_York'::timestamptz at time zone
    'UTC';

          timezone       

    ---------------------

     2012-01-01 05:00:00

    
    Cheers,

    Steve