How to calculate the local datetime from a utc dat

Question

i want to loop over a period of time in tsql, and print the utc datetimes and our local variant. We live in UTC +1, so i could easily add 1 hour, but in the summertime we live in UTC +2.

In C# i can create a datetime and use a method to ask for the UTC variant and vice versa.

Till now i have this:

declare @counter int
declare @localdate datetime
declare @utcdate datetime
 set @counter = 0
 while @counter < 100
 begin
   set @counter = @counter + 1
   print 'The counter is ' + cast(@counter as char)
  set @utcdate  = DATEADD(day,@counter,GETUTCDATE())
  --set @localdate = ????

  print  @localdate  
  print @utcdate
 end

Answer 1

While the question's title mentions SQL Server 2005, the question is tagged with SQL Server in general. For SQL Server 2016 and later, you can use:

SELECT yourUtcDateTime AT TIME ZONE 'Mountain Standard Time'

A list of time zones is available with SELECT * FROM sys.time_zone_info

Answer 2

Bobman's answer is close, but has a couple bugs: 1) You must compare local DAYLIGHT time (instead of local STANDARD time) to the Daylight Saving End DateTime. 2) SQL BETWEEN is Inclusive so you should be comparing using ">= and <" instead of BETWEEN.

Here is a working modified version along with some test cases: (Again, this only works for United States)

-- Test cases:
-- select dbo.fn_utc_to_est_date('2016-03-13 06:59:00.000') -- -> 2016-03-13 01:59:00.000 (Eastern Standard Time)
-- select dbo.fn_utc_to_est_date('2016-03-13 07:00:00.000') -- -> 2016-03-13 03:00:00.000 (Eastern Daylight Time)
-- select dbo.fn_utc_to_est_date('2016-11-06 05:59:00.000') -- -> 2016-11-06 01:59:00.000 (Eastern Daylight Time)
-- select dbo.fn_utc_to_est_date('2016-11-06 06:00:00.000') -- -> 2016-11-06 01:00:00.000 (Eastern Standard Time)
CREATE FUNCTION [dbo].[fn_utc_to_est_date]
(
    @utc datetime
)
RETURNS datetime
as
begin
    -- set offset in standard time (WITHOUT daylight saving time)
    declare @offset smallint
    set @offset = -5  --EST

    declare @localStandardTime datetime
    SET @localStandardTime = dateadd(hh, @offset, @utc)

    -- DST in USA starts on the second sunday of march and ends on the first sunday of november.
    -- DST was extended beginning in 2007:
    --   https://en.wikipedia.org/wiki/Daylight_saving_time_in_the_United_States#Second_extension_.282005.29
    -- If laws/rules change, obviously the below code needs to be updated.

    declare @dstStartDate datetime,
            @dstEndDate datetime,
            @year int
    set @year = datepart(year, @localStandardTime)

    -- get the first possible DST start day
    if (@year > 2006) set @dstStartDate = cast(@year as char(4)) + '-03-08 02:00:00'
    else              set @dstStartDate = cast(@year as char(4)) + '-04-01 02:00:00'
    while ((datepart(weekday,@dstStartDate) != 1)) begin --while not sunday
        set @dstStartDate = dateadd(day, 1, @dstStartDate)
    end

    -- get the first possible DST end day
    if (@year > 2006) set @dstEndDate = cast(@year as char(4)) + '-11-01 02:00:00'
    else              set @dstEndDate = cast(@year as char(4)) + '-10-25 02:00:00'
    while ((datepart(weekday,@dstEndDate) != 1)) begin --while not sunday
        set @dstEndDate = dateadd(day, 1, @dstEndDate)
    end

    declare @localTimeFinal datetime,
            @localTimeCompare datetime
    -- if local date is same day as @dstEndDate day,
    -- we must compare the local DAYLIGHT time to the @dstEndDate (otherwise we compare using local STANDARD time).
    -- See: http://www.timeanddate.com/time/change/usa?year=2016
    if (datepart(month,@localStandardTime) = datepart(month,@dstEndDate)
            and datepart(day,@localStandardTime) = datepart(day,@dstEndDate)) begin
        set @localTimeCompare = dateadd(hour, 1, @localStandardTime)
    end
    else begin
        set @localTimeCompare = @localStandardTime
    end

    set @localTimeFinal = @localStandardTime

    -- check for DST
    if (@localTimeCompare >= @dstStartDate and @localTimeCompare < @dstEndDate) begin
        set @localTimeFinal = dateadd(hour, 1, @localTimeFinal)
    end

    return @localTimeFinal
end

Answer 3

For those stuck in SQL Server 2005 and don't want or can't use a udf - and particularly does outside of the USA - I've taken @Bobman's approach and generalized it. The following will work in the USA, Europe, New Zealand and Australia, with the caveat that not all Australian states observe DST, even states that are in the same "base" timezone. It's also easy to add DST-rules that aren't yet supported, just add a line to the @calculation values.

-- =============================================
-- Author:      Herman Scheele
-- Create date: 20-08-2016
-- Description: Convert UTC datetime to local datetime
--              based on server time-distance from utc.
-- =============================================
create function dbo.UTCToLocalDatetime(@UTCDatetime datetime)
returns datetime as begin
    declare @LocalDatetime datetime, @DSTstart datetime, @DSTend datetime

    declare @calculation table (
        frm smallint,
        til smallint,
        since smallint,
        firstPossibleStart datetime,-- Put both of these with local non-DST time!
          firstPossibleEnd datetime -- (In Europe we turn back the clock from 3 AM to 2 AM, which means it happens 2 AM non-DST time)
    )

    insert into @calculation
    values
        (-9, -2, 1967, '1900-04-24 02:00', '1900-10-25 01:00'), -- USA first DST implementation
        (-9, -2, 1987, '1900-04-01 02:00', '1900-10-25 01:00'), -- USA first DST extension
        (-9, -2, 2007, '1900-03-08 02:00', '1900-11-01 01:00'), -- USA second DST extension
        (-1,  3, 1900, '1900-03-25 02:00', '1900-10-25 02:00'), -- Europe
        (9.5,11, 1971, '1900-10-01 02:00', '1900-04-01 02:00'), -- Australia (not all Aus states in this time-zone have DST though)
        (12, 13, 1974, '1900-09-24 02:00', '1900-04-01 02:00')  -- New Zealand

    select top 1    -- Determine if it is DST /right here, right now/ (regardless of input datetime)
        @DSTstart = dateadd(year, datepart(year, getdate())-1900, firstPossibleStart),          -- Grab first possible Start and End of DST period
        @DSTend   = dateadd(year, datepart(year, getdate())-1900, firstPossibleEnd),            
        @DSTstart = dateadd(day, 6 - (datepart(dw, @DSTstart) + @@datefirst - 2) % 7, @DSTstart),-- Shift Start and End of DST to first sunday
        @DSTend   = dateadd(day, 6 - (datepart(dw, @DSTend) + @@datefirst - 2) % 7, @DSTend),
        @LocalDatetime = dateadd(hour, datediff(hour, getutcdate(), getdate()), @UTCDatetime),  -- Add hours to current input datetime (including possible DST hour)
        @LocalDatetime = case
                when frm < til and getdate() >= @DSTstart and getdate() < @DSTend               -- If it is currently DST then we just erroneously added an hour above,
                  or frm > til and (getdate() >= @DSTstart or getdate() < @DSTend)              -- substract 1 hour to get input datetime in current non-DST timezone,
                    then dateadd(hour, -1, @LocalDatetime)                                      -- regardless of whether it is DST on the date of the input datetime
                else @LocalDatetime
            end
    from @calculation
    where datediff(minute, getutcdate(), getdate()) between frm * 60 and til * 60
      and datepart(year, getdate()) >= since
    order by since desc

    select top 1    -- Determine if it was/will be DST on the date of the input datetime in a similar fashion
        @DSTstart = dateadd(year, datepart(year, @LocalDatetime)-1900, firstPossibleStart),
        @DSTend   = dateadd(year, datepart(year, @LocalDatetime)-1900, firstPossibleEnd),
        @DSTstart = dateadd(day, 6 - (datepart(dw, @DSTstart) + @@datefirst - 2) % 7, @DSTstart),
        @DSTend   = dateadd(day, 6 - (datepart(dw, @DSTend) + @@datefirst - 2) % 7, @DSTend),
        @LocalDatetime = case
                when frm < til and @LocalDatetime >= @DSTstart and @LocalDatetime < @DSTend     -- If it would be DST on the date of the input datetime,
                  or frm > til and (@LocalDatetime >= @DSTstart or @LocalDatetime < @DSTend)    -- add this hour to the input datetime.
                    then dateadd(hour, 1, @LocalDatetime)
                else @LocalDatetime
            end
    from @calculation
    where datediff(minute, getutcdate(), getdate()) between frm * 60 and til * 60
      and datepart(year, @LocalDatetime) >= since
    order by since desc

    return @LocalDatetime
end

This function looks at the difference between local and utc time at the moment it runs to determine which DST-rules to apply. It then knows whether doing datediff(hour, getutcdate(), getdate()) includes a DST hour or not and subtracts it if it does. Then it determines whether it was or will be DST at the date of the input UTC datetime and if so adds the DST hour back.

This comes with one quirk, which is that during the last hour of DST and the first hour of non-DST, the function has no way of determining which it is and assumes the latter. So regardless of input-datetime, if this codes runs during the last hour of DST it will give the wrong outcome. Which means this works 99.9886% of the time.

Answer 4

Assuming you are using SQL 2005 upwards, you can develop a SQL CLR function to take a UTC date and convert to the local date.

This link is an MSDN How-To explaining how you can create a scalar UDF in C#.

Create a SQL function along the lines of

[SqlFunction()]
public static SqlDateTime ConvertUtcToLocal(SqlDateTime utcDate) 
{
    // over to you to convert SqlDateTime to DateTime, specify Kind
    // as UTC, convert to local time, and convert back to SqlDateTime
}

Your sample above would then become

set @localdate = dbo.ConvertUtcToLocal(@utcdate)

SQL CLR has its overheads in terms of deployment, but I feel cases like this are where it fits in best.

Answer 5

GETUTCDATE() just gives you the current time in UTC, any DATEADD() you do to this value will not include any daylight savings time shifts.

Your best bet is build your own UTC conversion table or just use something like this:

http://www.codeproject.com/KB/database/ConvertUTCToLocal.aspx

Answer 6

Here is a function (again US ONLY) but it is a bit more flexible. It will convert a UTC date to the server local time. It starts by adjusting the appointment date based on the current offset and then adjusts based on the difference of the current offset and the offset of the date of the appointment.

CREATE FUNCTION [dbo].[fnGetServerTimeFromUTC]
(
    @AppointmentDate AS DATETIME, 
    @DateTimeOffset DATETIMEOFFSET
)
RETURNS DATETIME
AS
BEGIN
    --DECLARE @AppointmentDate DATETIME;
    --SET @AppointmentDate = '2016-12-01 12:00:00'; SELECT @AppointmentDate;

    --Get DateTimeOffset from Server
    --DECLARE @DateTimeOffset; SET @DateTimeOffset = SYSDATETIMEOFFSET();
    DECLARE @DateTimeOffsetStr NVARCHAR(34) = @DateTimeOffset;

    --Set a standard DatePart value for Sunday (server configuration agnostic)
    DECLARE @dp_Sunday INT = 7 - @@DATEFIRST + 1;

    --2006 DST Start First Sunday in April (earliest is 04-01) Ends Last Sunday in October (earliest is 10-25)
    --2007 DST Start Second Sunday March (earliest is 03-08) Ends First Sunday Nov (earliest is 11-01)
    DECLARE @Start2006 NVARCHAR(6) = '04-01-';
    DECLARE @End2006 NVARCHAR(6) = '10-25-';
    DECLARE @Start2007 NVARCHAR(6) = '03-08-';
    DECLARE @End2007 NVARCHAR(6) = '11-01-';

    DECLARE @ServerDST SMALLINT = 0;
    DECLARE @ApptDST SMALLINT = 0;
    DECLARE @Start DATETIME;
    DECLARE @End DATETIME;

    DECLARE @CurrentMinuteOffset INT; 

    DECLARE @str_Year NVARCHAR(4) = LEFT(@DateTimeOffsetStr,4);
    DECLARE @Year INT = CONVERT(INT, @str_Year);

    SET @CurrentMinuteOffset = CONVERT(INT, SUBSTRING(@DateTimeOffsetStr,29,3)) * 60 + CONVERT(INT, SUBSTRING(@DateTimeOffsetStr,33,2)); --Hours + Minutes

    --Determine DST Range for Server Offset
    SET @Start = CASE 
        WHEN @Year <= 2006 THEN CONVERT(DATETIME, @Start2006 + @str_Year + ' 02:00:00')
        ELSE CONVERT(DATETIME, @Start2007 + @str_Year + ' 02:00:00')
        END;
    WHILE @dp_Sunday <> DATEPART(WEEKDAY, @Start) BEGIN
        SET @Start = DATEADD(DAY, 1, @Start)
    END;

    SET @End = CASE 
        WHEN @Year <= 2006 THEN CONVERT(DATETIME, @End2006 + @str_Year + ' 02:00:00')
        ELSE CONVERT(DATETIME, @End2007 + @str_Year + ' 02:00:00')
        END;
    WHILE @dp_Sunday <> DATEPART(WEEKDAY, @End) BEGIN
        SET @End = DATEADD(DAY, 1, @End)
    END;

    --Determine Current Offset based on Year
    IF @DateTimeOffset >= @Start AND @DateTimeOffset < @End SET @ServerDST = 1;

    --Determine DST status of Appointment Date
    SET @Year = YEAR(@AppointmentDate);

    SET @Start = CASE 
        WHEN @Year <= 2006 THEN CONVERT(DATETIME, @Start2006 + @str_Year + ' 02:00:00')
        ELSE CONVERT(DATETIME, @Start2007 + @str_Year + ' 02:00:00')
        END;
    WHILE @dp_Sunday <> DATEPART(WEEKDAY, @Start) BEGIN
        SET @Start = DATEADD(DAY, 1, @Start)
    END;

    SET @End = CASE 
        WHEN @Year <= 2006 THEN CONVERT(DATETIME, @End2006 + @str_Year + ' 02:00:00')
        ELSE CONVERT(DATETIME, @End2007 + @str_Year + ' 02:00:00')
        END;
    WHILE @dp_Sunday <> DATEPART(WEEKDAY, @End) BEGIN
        SET @End = DATEADD(DAY, 1, @End)
    END;

    --Determine Appointment Offset based on Year
    IF @AppointmentDate >= @Start AND @AppointmentDate < @End SET @ApptDST = 1;

    SET @AppointmentDate = DATEADD(MINUTE, @CurrentMinuteOffset + 60 * (@ApptDST - @ServerDST), @AppointmentDate)

    RETURN @AppointmentDate
END
GO