How to create Daylight Savings time Start and End function in SQL Server

StackOverflow https://stackoverflow.com/questions/19732896

Question

I need to create a function in SQL server that returns daylight savings time start datetime and daylight savings time end datetime.

I've come across a few examples on the web, however they all are using the 1st date of March and the 1st date of November and thats not technically correct.

Daylight savings time begins at 2AM on the 2nd Sunday of March and ends on at 2AM in the first Sunday in November.

I've started with the below code but I'm sure its wrong. Any assistance is appreciated! :)

DECLARE @DSTSTART DATETIME

SELECT @DSTSTART = CASE WHEN 
DATEPART(MONTH, SYSDATETIME()) = 3
AND DATEPART(weekday, SYSDATETIME()) = 1
AND DATEDIFF(week,dateadd(week, datediff(week, 0, dateadd(month, datediff(month, 0, SYSDATETIME()), 0)), 0), SYSDATETIME() - 1) = 2
AND DATEPART(HOUR, SYSDATETIME()) = 2
THEN SYSDATETIME()
END
RETURN (@DSTSTART)
END
GO
Was it helpful?

Solution

As pointed out in comments, right now (March 2022) this calculation looks likely to change next year: US may not switch off of DST in the fall.

Don't forget that daylight saving time schedules change depending on country, and also are subject to change over the years: the current (as of 2013 through 2022) US system took effect in 2007, for example.

Assuming you want the current system for the US, here's one form of an answer for any given year.

SET DATEFIRST 7

DECLARE @year INT = 2013
DECLARE
    @StartOfMarch DATETIME ,
    @StartOfNovember DATETIME ,
    @DstStart DATETIME ,
    @DstEnd DATETIME


  
SET @StartOfMarch = DATEADD(MONTH, 2, DATEADD(YEAR, @year - 1900, 0))
SET @StartOfNovember = DATEADD(MONTH, 10, DATEADD(YEAR, @year - 1900, 0));
SET @DstStart = DATEADD(HOUR, 2,
                        DATEADD(day,
                                ( ( 15 - DATEPART(dw, @StartOfMarch) ) % 7 )
                                + 7, @StartOfMarch))
SET @DstEnd = DATEADD(HOUR, 2,
                      DATEADD(day,
                              ( ( 8 - DATEPART(dw, @StartOfNovember) ) % 7 ),
                              @StartOfNovember))


SELECT
    @DstStart AS DstStartInUS ,
    @DstEnd AS DstEndInUS

or as functions, but you have to know that DateFirst is set to 7, otherwise the math will be off.

CREATE FUNCTION GetDstStart ( @Year AS INT )
RETURNS DATETIME
AS
    BEGIN

        DECLARE
            @StartOfMarch DATETIME ,
            @DstStart DATETIME 

        SET @StartOfMarch = DATEADD(MONTH, 2,
                                    DATEADD(YEAR, @year - 1900, 0))
        SET @DstStart = DATEADD(HOUR, 2,
                                DATEADD(day,
                                        ( ( 15 - DATEPART(dw,
                                                          @StartOfMarch) )
                                          % 7 ) + 7, @StartOfMarch))
        RETURN @DstStart
    END

GO;


CREATE FUNCTION GetDstEnd ( @Year AS INT )
RETURNS DATETIME
AS
    BEGIN
        DECLARE
            @StartOfNovember DATETIME ,
            @DstEnd DATETIME

        SET @StartOfNovember = DATEADD(MONTH, 10,
                                       DATEADD(YEAR, @year - 1900, 0))
        SET @DstEnd = DATEADD(HOUR, 2,
                              DATEADD(day,
                                      ( ( 8 - DATEPART(dw,
                                                       @StartOfNovember) )
                                        % 7 ), @StartOfNovember))
        RETURN @DstEnd
    END

OTHER TIPS

Personally, I think it's easier to find the first Sunday in November than it is to find the second Sunday in March. Luckily, if you find one, you can find the other because there's always 238 days between them. So here's a handy function to find the end of Dst:

create function GetDstEnd (
                           @Year int
                          )
returns datetime
as
begin

   declare @DstEnd datetime;

   ;with FirstWeekOfNovember
   as (
       select top(7)
              cast(@Year as char(4))
            + '-11-0'
            + cast(row_number() over(order by object_id) as char(1))
            + ' 02:00:00'
              'DST_Stops'
         from sys.columns
      )
   select @DstEnd = DST_Stops
     from FirstWeekOfNovember
    where datepart(weekday,DST_Stops) = 1

   return @DstEnd;

end;

Now the Start of Dst is the same function, only 238 days earlier.

create function GetDstStart (
                             @Year int
                            )
returns datetime
as
begin;

   declare @DstStart datetime;

   ;with FirstWeekOfNovember
   as (
       select top(7)
              cast(@Year as char(4))
            + '-11-0'
            + cast(row_number() over(order by object_id) as char(1))
            + ' 02:00:00'
              'DST_Stops'
         from sys.columns
      )
   select @DstStart = dateadd(day,-238,DST_Stops)
     from FirstWeekOfNovember
    where datepart(weekday,DST_Stops) = 1

   return @DstStart;

end;
go

SQL Server version 2016 will solve this issue once and for all. For earlier versions a CLR solution is probably easiest. Or for a specific DST rule (like US only), a T-SQL function can be relatively simple.

However, I think a generic T-SQL solution might be possible. As long as xp_regread works, try this:

CREATE TABLE #tztable (Value varchar(50), Data binary(56));
DECLARE @tzname varchar(150) = 'SYSTEM\CurrentControlSet\Control\TimeZoneInformation'
EXEC master.dbo.xp_regread 'HKEY_LOCAL_MACHINE', @tzname, 'TimeZoneKeyName', @tzname OUT;
SELECT @tzname = 'SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\' + @tzname
INSERT INTO #tztable
EXEC master.dbo.xp_regread 'HKEY_LOCAL_MACHINE', @tzname, 'TZI';
SELECT                                                                                  -- See http://msdn.microsoft.com/ms725481
 CAST(CAST(REVERSE(SUBSTRING(Data,  1, 4)) AS binary(4))      AS int) AS BiasMinutes,   -- UTC = local + bias: > 0 in US, < 0 in Europe!
 CAST(CAST(REVERSE(SUBSTRING(Data,  5, 4)) AS binary(4))      AS int) AS ExtraBias_Std, --   0 for most timezones
 CAST(CAST(REVERSE(SUBSTRING(Data,  9, 4)) AS binary(4))      AS int) AS ExtraBias_DST, -- -60 for most timezones: DST makes UTC 1 hour earlier
 -- When DST ends:
 CAST(CAST(REVERSE(SUBSTRING(Data, 13, 2)) AS binary(2)) AS smallint) AS StdYear,       -- 0 = yearly (else once)
 CAST(CAST(REVERSE(SUBSTRING(Data, 15, 2)) AS binary(2)) AS smallint) AS StdMonth,      -- 0 = no DST
 CAST(CAST(REVERSE(SUBSTRING(Data, 17, 2)) AS binary(2)) AS smallint) AS StdDayOfWeek,  -- 0 = Sunday to 6 = Saturday
 CAST(CAST(REVERSE(SUBSTRING(Data, 19, 2)) AS binary(2)) AS smallint) AS StdWeek,       -- 1 to 4, or 5 = last <DayOfWeek> of <Month>
 CAST(CAST(REVERSE(SUBSTRING(Data, 21, 2)) AS binary(2)) AS smallint) AS StdHour,       -- Local time
 CAST(CAST(REVERSE(SUBSTRING(Data, 23, 2)) AS binary(2)) AS smallint) AS StdMinute,
 CAST(CAST(REVERSE(SUBSTRING(Data, 25, 2)) AS binary(2)) AS smallint) AS StdSecond,
 CAST(CAST(REVERSE(SUBSTRING(Data, 27, 2)) AS binary(2)) AS smallint) AS StdMillisec,
 -- When DST starts:
 CAST(CAST(REVERSE(SUBSTRING(Data, 29, 2)) AS binary(2)) AS smallint) AS DSTYear,       -- See above
 CAST(CAST(REVERSE(SUBSTRING(Data, 31, 2)) AS binary(2)) AS smallint) AS DSTMonth,
 CAST(CAST(REVERSE(SUBSTRING(Data, 33, 2)) AS binary(2)) AS smallint) AS DSTDayOfWeek,
 CAST(CAST(REVERSE(SUBSTRING(Data, 35, 2)) AS binary(2)) AS smallint) AS DSTWeek,
 CAST(CAST(REVERSE(SUBSTRING(Data, 37, 2)) AS binary(2)) AS smallint) AS DSTHour,
 CAST(CAST(REVERSE(SUBSTRING(Data, 39, 2)) AS binary(2)) AS smallint) AS DSTMinute,
 CAST(CAST(REVERSE(SUBSTRING(Data, 41, 2)) AS binary(2)) AS smallint) AS DSTSecond,
 CAST(CAST(REVERSE(SUBSTRING(Data, 43, 2)) AS binary(2)) AS smallint) AS DSTMillisec
FROM #tztable;
DROP TABLE #tztable

A (complex) T-SQL function could use this data to determine the exact offset for all dates during the current DST rule.

I wasn't really satisfied with any of the solutions I found online to convert UTC to local time, so I came up with this function. Have a look at my SO answer here

There is some logic in there that calculates whether daylight savings is active based on the standard date range DST uses (Second Sunday in March at 2am, clocks move forward; 1st Sunday in November revert to standard time)

FWIW, there is much easier option starting with SQL Server 2016. System table 'sys.time_zone_info' has the offsets and DST flag to determine how to convert to the desired timezone. For more information about this table, please see the MS Doc.

You can identify the version of your SQL Server by this simple query: SELECT @@VERSION;

You can identify the timezone your SQL Server is in by this simple query: SELECT CURRENT_TIMEZONE();

You should be able to put together these resources to convert datetime values to approp. timezone while respecting the DST rules.

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