Effectively Converting dates between UTC and Local (ie. PST) time in SQL 2005

If you're in the US and only interested in going from UTC/GMT to a fixed time zone (such as EDT) this code should suffice. I whipped it up today and believe it's correct but use at your own risk.

Adds a computed column to a table 'myTable' assuming your dates are on the 'date' column. Hope someone else finds this useful.

ALTER TABLE myTable ADD date_edt AS 
  dateadd(hh, 
        -- The schedule through 2006 in the United States was that DST began on the first Sunday in April 
        -- (April 2, 2006), and changed back to standard time on the last Sunday in October (October 29, 2006). 
        -- The time is adjusted at 02:00 local time.
              CASE WHEN YEAR(date) <= 2006 THEN  
                    CASE WHEN 
                              date >=  '4/' + CAST(abs(8-DATEPART(dw,'4/1/' + CAST(YEAR(date) as varchar)))%7 + 1 as varchar) +  '/' + CAST(YEAR(date) as varchar) + ' 2:00' 
                          AND 
                              date < '10/' + CAST(32-DATEPART(dw,'10/31/' + CAST(YEAR(date) as varchar)) as varchar) +  '/' + CAST(YEAR(date) as varchar) + ' 2:00' 
                    THEN -4 ELSE -5 END
              ELSE
        -- By the Energy Policy Act of 2005, daylight saving time (DST) was extended in the United States in 2007. 
        -- DST starts on the second Sunday of March, which is three weeks earlier than in the past, and it ends on 
        -- the first Sunday of November, one week later than in years past. This change resulted in a new DST period 
        -- that is four weeks (five in years when March has five Sundays) longer than in previous years.[35] In 2008 
        -- daylight saving time ended at 02:00 on Sunday, November 2, and in 2009 it began at 02:00 on Sunday, March 8.[36]
                    CASE WHEN 
                              date >= '3/' + CAST(abs(8-DATEPART(dw,'3/1/' + CAST(YEAR(date) as varchar)))%7 + 8 as varchar) +  '/' + CAST(YEAR(date) as varchar) + ' 2:00' 
                          AND 
                              date < 
                                '11/' + CAST(abs(8-DATEPART(dw,'11/1/' + CAST(YEAR(date) as varchar)))%7 + 1 as varchar) +  '/' + CAST(YEAR(date) as varchar) + ' 2:00' 
                    THEN -4 ELSE -5 END
              END
  ,date)

Create two tables and then join to them to convert stored GMT dates to local time:

TimeZones     e.g.
---------     ----
TimeZoneId    19
Name          Eastern (GMT -5)
Offset        -5

Create the daylight savings table and populate it with as much information as you can (local laws change all the time so there's no way to predict what the data will look like years in the future)

DaylightSavings
---------------
TimeZoneId    19
BeginDst      3/9/2008 2:00 AM
EndDst        11/2/2008 2:00 AM

Join them like this:

inner join  TimeZones       tz on x.TimeZoneId=tz.TimeZoneId
left join   DaylightSavings ds on tz.TimeZoneId=ds.LocalTimeZone 
    and x.TheDateToConvert between ds.BeginDst and ds.EndDst

Convert dates like this:

dateadd(hh, tz.Offset + 
    case when ds.LocalTimeZone is not null 
    then 1 else 0 end, TheDateToConvert)

FOR READ-ONLY Use this(inspired by Bob Albright's incorrect solution ):

SELECT
  date1, 
  dateadd(hh,
    -- The schedule through 2006 in the United States was that DST began on the first Sunday in April 
    -- (April 2, 2006), and changed back to standard time on the last Sunday in October (October 29, 2006). 
    -- The time is adjusted at 02:00 local time (which, for edt, is 07:00 UTC at the start, and 06:00 GMT at the end).
    CASE WHEN YEAR(date1) <= 2006 THEN
         CASE WHEN 
                  date1 >=  '4/' + CAST((8-DATEPART(dw,'4/1/' + CAST(YEAR(date1) as varchar)))%7 + 1 as varchar) +  '/' + CAST(YEAR(date1) as varchar) + ' 7:00' 
                AND 
                  date1 < '10/' + CAST(32-DATEPART(dw,'10/31/' + CAST(YEAR(date1) as varchar)) as varchar) +  '/' + CAST(YEAR(date1) as varchar) + ' 6:00' 
              THEN -4 ELSE -5 END
    ELSE
        -- By the Energy Policy Act of 2005, daylight saving time (DST) was extended in the United States in 2007. 
        -- DST starts on the second Sunday of March, which is three weeks earlier than in the past, and it ends on 
        -- the first Sunday of November, one week later than in years past. This change resulted in a new DST period 
        -- that is four weeks (five in years when March has five Sundays) longer than in previous years. In 2008 
        -- daylight saving time ended at 02:00 edt (06:00 UTC) on Sunday, November 2, and in 2009 it began at 02:00 edt (07:00 UTC) on Sunday, March 8
        CASE WHEN 
                 date1 >= '3/' + CAST((8-DATEPART(dw,'3/1/' + CAST(YEAR(date1) as varchar)))%7 + 8 as varchar) +  '/' + CAST(YEAR(date1) as varchar) + ' 7:00' 
               AND 
                 date1 < '11/' + CAST((8-DATEPART(dw,'11/1/' + CAST(YEAR(date1) as varchar)))%7 + 1 as varchar) +  '/' + CAST(YEAR(date1) as varchar) + ' 6:00' 
             THEN -4 ELSE -5 END
    END
   , date1) as date1Edt
  from MyTbl

I posted this answer after I tried to edit Bob Albright's wrong answer. I corrected the times and removed superfluous abs(), but my edits were rejected multiple times. I tried explaining, but was dismissed as a noob. His is a GREAT approach to the problem! It got me started in the right direction. I hate to create this separate answer when his just needs a minor tweak, but I tried ¯\_(ツ)_/¯

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Sql Server

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