Get daylight savings time

,

 

 

Hello everyone,

 

Many of us have come across cases where we need to convert a time (such as UTC) to local time, but what about those parts of the world where daylight savings time is observed?

 

This function will return the daylight savings date/time for a given year in accordance with historical records: http://www.energy.ca.gov/daylightsaving.html#chart

 

This way, no lookup tables are needed. All you have to know is what timezone you are converting from and to, because as long as you have the date when daylight savings begins/ends, then it should be fairly easy to write some case logic to convert the date. This script also contains an example of this function's use by converting UTC to PST.

 

Caveat: The function can return undesirable results if the SET DATEFIRST statement is set to anything other than Sunday.

 

Credits: Much credit to those out there who provided much of the crucial code that went into building this function.

if not object_id('dbo.udf_GetDaylightSavingsTime') is null
    drop function dbo.udf_GetDaylightSavingsTime
GO

CREATE function [dbo].udf_GetDaylightSavingsTime(@year int, @extent varchar(255))
returns datetime
/**********************************************************************
PROCEDURE:      udf_GetDaylightSavingsTime
PARAMETERS:     @year: the year to return the daylight savings time begin
                date
                
                @extent ('Begin' or 'End'): indicates whether the function
                returns the begin or end date of daylight savings time

APPLICATION:	System Support
PURPOSE:		This procedure will return the date of daylight savings
                time according to the current federal schedule, which is 
                currently the second Sunday in March: http://www.energy.ca.gov/daylightsaving.html#chart

NOTES:          This procedure will also return the begin date for the previous
                schedule which ended in 2006, which was the first Sunday of 
                April

EXAMPLES:       print dbo.udf_GetDaylightSavingsTime(2003, 'Begin')
                print dbo.udf_GetDaylightSavingsTime(2003, 'End')                
                
                print dbo.udf_GetDaylightSavingsTime(2009, 'Begin')
                print dbo.udf_GetDaylightSavingsTime(2009, 'end')
                
MODIFIED DATE   AUTHOR              DESCRIPTION
--------------  --------------      -------------------------------

**********************************************************************/
as
begin -- function

declare @dateTime datetime
,       @ErrorMessage varchar(1000)
,       @ProcName     varchar(128)

SET @ProcName = object_name(@@procid)

if @extent not in('Begin', 'End')
begin
	set @dateTime = 1/0
end

set @dateTime = case @extent
                    when 'Begin' then
                        case 
                            --latest daylight savings time 
                            when @year >= 2007 then cast('3/8/' + cast(@year as varchar(4)) as datetime)
                            --old daylight savings time prior to 2007
                            when @year <= 2006 then cast('4/1/' + cast(@year as varchar(4)) as datetime)
                        end
                    when 'End' then
                        case 
                            --latest daylight savings time 
                            when @year >= 2007 then cast('11/1/' + cast(@year as varchar(4)) as datetime)
                            --old daylight savings time prior to 2007
                            when @year <= 2006 then cast('10/31/' + cast(@year as varchar(4)) as datetime)
                        end
                end

set @dateTime = case
                    when @extent = 'End' and @year <= 2006
                        then DATEADD(DAY,1-DATEPART(weekday,dateadd(ms,-3,DATEADD(mm, DATEDIFF(m,0,@dateTime)+1, 0))),dateadd(ms,-3,DATEADD(mm, DATEDIFF(m,0,@dateTime)+1, 0)))
                else
                    case when datepart(dw, @dateTime) = 1
                        then @dateTime
                        else dateadd(dd, 8 - datepart(dw, @dateTime), @dateTime)
                    end
                end
                
-- daylight savings time begins at 2 am
return convert(varchar(10), @dateTime, 101) + ' 02:00:00'
end -- function
GO



declare @TestyWesty table (UTCDate datetime)

--Test DST Begin date
insert into @TestyWesty values('03/08/2009 09:59:59')
insert into @TestyWesty values('03/08/2009 10:00:00')
insert into @TestyWesty values('03/08/2009 10:00:01')

--Test DST End date
insert into @TestyWesty values('11/01/2009 08:59:59')
insert into @TestyWesty values('11/01/2009 09:00:00')
insert into @TestyWesty values('11/01/2009 09:00:01')

SELECT 
      case
        when dateadd(hour, -8, UTCDate) >= dbo.udf_GetDaylightSavingsTime(year(UTCDate), 'Begin') and dateadd(hour, -7, UTCDate) < dbo.udf_GetDaylightSavingsTime(year(UTCDate), 'End')
            then dateadd(hour, -7, UTCDate)
        else
            dateadd(hour, -8, UTCDate)
       end as UTCDate
from   @TestyWesty

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