SQL Server - Convert date field to UTC

Question

I have recently updated my system to record date/times as UTC as previously they were storing as local time.

I now need to convert all the local stored date/times to UTC. I was wondering if there is any built in function, similar to .NET's ConvertTime method?

I am trying to avoid having to write a utility app to do this for me.

Any suggestions?

Answer 1

I do not believe the above code will work. The reason is that it depends upon the difference between the current date in local and UTC times. For example, here in California we are now in PDT (Pacific Daylight Time); the difference between this time and UTC is 7 hours. The code provided will, if run now, add 7 hours to every date which is desired to be converted. But if a historical stored date, or a date in the future, is converted, and that date is not during daylight savings time, it will still add 7, when the correct offset is 8. Bottom line: you cannot convert date/times properly between time zones (including UTC, which does not obey daylight savings time) by only looking at the current date. You must consider the date itself that you are converting, as to whether daylight time was in force on that date. Furthermore, the dates at which daylight and standard times change themselves have changed (George Bush changed the dates during his administration for the USA!). In other words, any solution which even references getdate() or getutcdate() does not work. It must parse the actual date to be converted.

Answer 2

We can convert ServerZone DateTime to UTC and UTC to ServerZone DateTime

Simply run the following scripts to understand the conversion then modify as what you need

--Get Server's TimeZone
DECLARE @ServerTimeZone VARCHAR(50)
EXEC MASTER.dbo.xp_regread 'HKEY_LOCAL_MACHINE',
'SYSTEM\CurrentControlSet\Control\TimeZoneInformation',
'TimeZoneKeyName',@ServerTimeZone OUT

-- ServerZone to UTC DATETIME
DECLARE @CurrentServerZoneDateTime DATETIME = GETDATE()
DECLARE @UTCDateTime  DATETIME =  @CurrentServerZoneDateTime AT TIME ZONE @ServerTimeZone AT TIME ZONE 'UTC' 
--(OR)
--DECLARE @UTCDateTime  DATETIME = GETUTCDATE()
SELECT @CurrentServerZoneDateTime AS CURRENTZONEDATE,@UTCDateTime AS UTCDATE

-- UTC to ServerZone DATETIME
SET @CurrentServerZoneDateTime = @UTCDateTime AT TIME ZONE 'UTC' AT TIME ZONE @ServerTimeZone
SELECT @UTCDateTime AS UTCDATE,@CurrentServerZoneDateTime AS CURRENTZONEDATE

Note: This(AT TIME ZONE) working on only SQL Server 2016+ and this advantage is automatically considering Daylight while converting to particular Time zone

Answer 3

Here's my quick and dirty version. I know all of my dates were using the US Eastern time zone. You can change the offset or otherwise make it smarter as you need to. I was doing a one-time migration so this was Good Enough.

CREATE FUNCTION [dbo].[ConvertToUtc]
(
    @date datetime
)
RETURNS DATETIME
AS
BEGIN
    -- Declare the return variable here
    DECLARE @utcDate datetime;
    DECLARE @offset int;

    SET @offset = (SELECT CASE WHEN 
                                    @date BETWEEN '1987-04-05 02:00 AM' AND '1987-10-25 02:00 AM'
                                 OR @date BETWEEN '1988-04-03 02:00 AM' AND '1988-10-30 02:00 AM'
                                 OR @date BETWEEN '1989-04-02 02:00 AM' AND '1989-10-29 02:00 AM'
                                 OR @date BETWEEN '1990-04-01 02:00 AM' AND '1990-10-28 02:00 AM'
                                 OR @date BETWEEN '1991-04-07 02:00 AM' AND '1991-10-27 02:00 AM'
                                 OR @date BETWEEN '1992-04-05 02:00 AM' AND '1992-10-25 02:00 AM'
                                 OR @date BETWEEN '1993-04-04 02:00 AM' AND '1993-10-31 02:00 AM'
                                 OR @date BETWEEN '1994-04-03 02:00 AM' AND '1994-10-30 02:00 AM'
                                 OR @date BETWEEN '1995-04-02 02:00 AM' AND '1995-10-29 02:00 AM'
                                 OR @date BETWEEN '1996-04-07 02:00 AM' AND '1996-10-27 02:00 AM'
                                 OR @date BETWEEN '1997-04-06 02:00 AM' AND '1997-10-26 02:00 AM'
                                 OR @date BETWEEN '1998-04-05 02:00 AM' AND '1998-10-25 02:00 AM'
                                 OR @date BETWEEN '1999-04-04 02:00 AM' AND '1999-10-31 02:00 AM'
                                 OR @date BETWEEN '2000-04-02 02:00 AM' AND '2000-10-29 02:00 AM'
                                 OR @date BETWEEN '2001-04-01 02:00 AM' AND '2001-10-28 02:00 AM'
                                 OR @date BETWEEN '2002-04-07 02:00 AM' AND '2002-10-27 02:00 AM'
                                 OR @date BETWEEN '2003-04-06 02:00 AM' AND '2003-10-26 02:00 AM'
                                 OR @date BETWEEN '2004-04-04 02:00 AM' AND '2004-10-31 02:00 AM'
                                 OR @date BETWEEN '2005-04-03 02:00 AM' AND '2005-10-30 02:00 AM'
                                 OR @date BETWEEN '2006-04-02 02:00 AM' AND '2006-10-29 02:00 AM'
                                 OR @date BETWEEN '2007-03-11 02:00 AM' AND '2007-11-04 02:00 AM'
                                 OR @date BETWEEN '2008-03-09 02:00 AM' AND '2008-11-02 02:00 AM'
                                 OR @date BETWEEN '2009-03-08 02:00 AM' AND '2009-11-01 02:00 AM'
                                 OR @date BETWEEN '2010-03-14 02:00 AM' AND '2010-11-07 02:00 AM'
                                 OR @date BETWEEN '2011-03-13 02:00 AM' AND '2011-11-06 02:00 AM'
                                 OR @date BETWEEN '2012-03-11 02:00 AM' AND '2012-11-04 02:00 AM'
                                 OR @date BETWEEN '2013-03-10 02:00 AM' AND '2013-11-03 02:00 AM'
                                 OR @date BETWEEN '2014-03-09 02:00 AM' AND '2014-11-02 02:00 AM'
                                 OR @date BETWEEN '2015-03-08 02:00 AM' AND '2015-11-01 02:00 AM'
                                 OR @date BETWEEN '2016-03-13 02:00 AM' AND '2016-11-06 02:00 AM'
                                 OR @date BETWEEN '2017-03-12 02:00 AM' AND '2017-11-05 02:00 AM'
                                 OR @date BETWEEN '2018-03-11 02:00 AM' AND '2018-11-04 02:00 AM'
                                 OR @date BETWEEN '2019-03-10 02:00 AM' AND '2019-11-03 02:00 AM'
                                 OR @date BETWEEN '2020-03-08 02:00 AM' AND '2020-11-01 02:00 AM'
                                 OR @date BETWEEN '2021-03-14 02:00 AM' AND '2021-11-07 02:00 AM'
                               THEN 4
                               ELSE 5 END);

    SELECT @utcDate = DATEADD(hh, @offset, @date)
    RETURN @utcDate;

END

Answer 4

With SQL Server 2016, there is now built-in support for time zones with the AT TIME ZONE statement. You can chain these to do conversions:

SELECT YourOriginalDateTime AT TIME ZONE 'Pacific Standard Time' AT TIME ZONE 'UTC'

Or, this would work as well:

SELECT SWITCHOFFSET(YourOriginalDateTime AT TIME ZONE 'Pacific Standard Time', '+00:00')

Either of these will interpret the input in Pacific time, properly account for whether or not DST is in effect, and then convert to UTC. The result will be a datetimeoffset with a zero offset.

More examples in the CTP announcement.

Answer 5

Depending on how far back you need to go, you can build a table of daylight savings times and then join the table and do a dst-sensitive conversion. This particular one converts from EST to GMT (i.e. uses offsets of 5 and 4).

select createdon, dateadd(hour, case when dstlow is null then 5 else 4 end, createdon) as gmt
from photos
left outer join (
          SELECT {ts '2009-03-08 02:00:00'} as dstlow, {ts '2009-11-01 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2010-03-14 02:00:00'} as dstlow, {ts '2010-11-07 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2011-03-13 02:00:00'} as dstlow, {ts '2011-11-06 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2012-03-11 02:00:00'} as dstlow, {ts '2012-11-04 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2013-03-10 02:00:00'} as dstlow, {ts '2013-11-03 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2014-03-09 02:00:00'} as dstlow, {ts '2014-11-02 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2015-03-08 02:00:00'} as dstlow, {ts '2015-11-01 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2016-03-13 02:00:00'} as dstlow, {ts '2016-11-06 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2017-03-12 02:00:00'} as dstlow, {ts '2017-11-05 02:00:00'} as dsthigh
UNION ALL SELECT {ts '2018-03-11 02:00:00'} as dstlow, {ts '2018-11-04 02:00:00'} as dsthigh
    ) dst
    on createdon >= dstlow and createdon < dsthigh

Answer 6

Unless I missed something above (possible), all of the methods above are flawed in that they don't take the overlap when switching from daylight savings (say EDT) to standard time (say EST) into account. A (very verbose) example:

[1] EDT 2016-11-06 00:59 - UTC 2016-11-06 04:59
[2] EDT 2016-11-06 01:00 - UTC 2016-11-06 05:00
[3] EDT 2016-11-06 01:30 - UTC 2016-11-06 05:30
[4] EDT 2016-11-06 01:59 - UTC 2016-11-06 05:59
[5] EST 2016-11-06 01:00 - UTC 2016-11-06 06:00
[6] EST 2016-11-06 01:30 - UTC 2016-11-06 06:30
[7] EST 2016-11-06 01:59 - UTC 2016-11-06 06:59
[8] EST 2016-11-06 02:00 - UTC 2016-11-06 07:00

Simple hour offsets based on date and time won't cut it. If you don't know if the local time was recorded in EDT or EST between 01:00 and 01:59, you won't have a clue! Let's use 01:30 for example: if you find later times in the range 01:31 through 01:59 BEFORE it, you won't know if the 01:30 you're looking at is [3 or [6. In this case, you can get the correct UTC time with a bit of coding be looking at previous entries (not fun in SQL), and this is the BEST case...

Say you have the following local times recorded, and didn't dedicate a bit to indicate EDT or EST:

                     UTC time         UTC time         UTC time
                     if [2] and [3]   if [2] and [3]   if [2] before
local time           before switch    after switch     and [3] after
[1] 2016-11-06 00:43     04:43         04:43           04:43
[2] 2016-11-06 01:15     05:15         06:15           05:15
[3] 2016-11-06 01:45     05:45         06:45           06:45
[4] 2016-11-06 03:25     07:25         07:25           07:25

Times [2] and [3] may be in the 5 AM timeframe, the 6 AM timeframe, or one in the 5 AM and the other in the 6 AM timeframe . . . In other words: you are hosed, and must throw out all readings between 01:00:00 and 01:59:59. In this circumstance, there is absolutely no way to resolve the actual UTC time!