How to calculate the local datetime from a utc datetime in tsql (sql 2005)?

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 i

Answer 1

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 2

You can use my SQL Server Time Zone Support project to convert between IANA standard time zones, as listed here.

Example:

SELECT Tzdb.UtcToLocal('2015-07-01 00:00:00', 'America/Los_Angeles')

Answer 3

This solution seems too obvious.

If you can get UTC Date with GETUTCDATE() and you can get your local date with GETDATE() you have an offset that you can apply for any datetime

SELECT DATEADD(hh, DATEPART(hh, GETDATE() - GETUTCDATE()) - 24, GETUTCDATE()) 

this should return the local time you executed the query,

SELECT DATEADD(hh, DATEPART(hh, GETDATE() - GETUTCDATE()) - 24, N'1/14/2011 7:00:00'  ) 

this will return 2011-01-14 02:00:00.000 because i'm in UTC +5

Unless I'm missing something?

Answer 4

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 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

I've been waiting for 5 years for a more elegant solution but since one has not emerged, I'll post what I've been using thus far...

CREATE FUNCTION [dbo].[UDTToLocalTime](@UDT AS DATETIME)  
RETURNS DATETIME
AS
BEGIN 
--====================================================
--Set the Timezone Offset (NOT During DST [Daylight Saving Time])
--====================================================
DECLARE @Offset AS SMALLINT
SET @Offset = -5

--====================================================
--Figure out the Offset Datetime
--====================================================
DECLARE @LocalDate AS DATETIME
SET @LocalDate = DATEADD(hh, @Offset, @UDT)

--====================================================
--Figure out the DST Offset for the UDT Datetime
--====================================================
DECLARE @DaylightSavingOffset AS SMALLINT
DECLARE @Year as SMALLINT
DECLARE @DSTStartDate AS DATETIME
DECLARE @DSTEndDate AS DATETIME
--Get Year
SET @Year = YEAR(@LocalDate)

--Get First Possible DST StartDay
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'
--Get DST StartDate 
WHILE (DATENAME(dw, @DSTStartDate) <> 'sunday') SET @DSTStartDate = DATEADD(day, 1,@DSTStartDate)


--Get First Possible DST EndDate
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'
--Get DST EndDate 
WHILE (DATENAME(dw, @DSTEndDate) <> 'sunday') SET @DSTEndDate = DATEADD(day,1,@DSTEndDate)

--Get DaylightSavingOffset
SET @DaylightSavingOffset = CASE WHEN @LocalDate BETWEEN @DSTStartDate AND @DSTEndDate THEN 1 ELSE 0 END

--====================================================
--Finally add the DST Offset 
--====================================================
RETURN DATEADD(hh, @DaylightSavingOffset, @LocalDate)
END



GO

Notes:

This is for North American servers that observer Daylight Saving Time. Please change the variable @Offest to the Timezone offset of the server running the SQL function (While NOT Observing the Daylight Savings time)...

--====================================================
--Set the Timezone Offset (NOT During DST [Daylight Saving Time])
--====================================================
DECLARE @Offset AS SMALLINT
SET @Offset = -5

As the DST rules change update them here...

--Get First Possible DST StartDay
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'
--Get DST StartDate 
WHILE (DATENAME(dw, @DSTStartDate) <> 'sunday') SET @DSTStartDate = DATEADD(day, 1,@DSTStartDate)


--Get First Possible DST EndDate
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'
--Get DST EndDate 
WHILE (DATENAME(dw, @DSTEndDate) <> 'sunday') SET @DSTEndDate = DATEADD(day,1,@DSTEndDate)

Cheers,