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

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