Convert to my time zone - SQL Server - sql

Question

Could somebody give me a help?
I have a source data where it is in another server in another country, when I get it to my B.I Staging area, I want to convert it to my time zone, but my SQL Server versions is 2014.
I can't use AT TIME ZONE SQL Server function.
Is there any other way to do this without fixing a hard coded value like below?
The below result doesn't solve my problem, because its hard coded values:
SELECT SWITCHOFFSET (DATETIMEFIELD, '-03:00')
SELECT TODATETIMEOFFSET(DATETIMEFIELD,'-03:00')
Ps: Not forgetting about DAYLIGHT SAVING

Answer 1

Without any sample data, this is very much a stab in the dark, that at least tries to explain it. Note, that this isn't fool proof. For example, when the clocks go back then the times 01:00:00 - 01:59:59 will occur twice. These times will always be assumed to be before DST, regardless of if they really were.
Anyway, firstly I created a small table with the dates that DST occurred both in the UK (GMT: UTC + 0) and in Eastern America (EST: UTC - 5).
Then, I created a query as below, using that data.
USE Sandbox;
GO
--SAmple Daylight Savings Table
CREATE TABLE DaylightSaving (Timezone char(3),
ClocksChangeDate datetime,
Change smallint,
UTCDiff smallint);
--Dates for UK
INSERT INTO DaylightSaving
VALUES ('GMT','20170326 02:00:00',1,0),
('GMT','20171029 02:00:00',0,0),
('GMT','20180325 02:00:00',1,0),
('GMT','20181028 02:00:00',0,0),
('GMT','20190331 02:00:00',1,0),
('GMT','20191027 02:00:00',0,0);
--Dates for Eastern America
INSERT INTO DaylightSaving
VALUES ('EST','20170312',1,-5),
('EST','20171105',0,-5),
('EST','20180311',1,-5),
('EST','20181104',0,-5),
('EST','20190310',1,-5),
('EST','20191103',0,-5);
GO
--Made up sample data
CREATE TABLE SampleData (DateAndTime datetime, Timezone char(3));
INSERT INTO SampleData
VALUES ('20180123 12:28:00.000','EST'),
('20180523 19:58:00.000','EST'),
('20181101 07:19:00.000','EST'),
('20190330 20:50:00.000','EST'),
('20190330 21:05:00.000','EST');
GO
SELECT DateAndTime AS ESTTime,
DATEADD(HOUR, 5 + DT.Change - ST.Change, DateAndTime) AS GMTTime,
DATEDIFF(HOUR, DateAndTime, DATEADD(HOUR, DT.UTCDiff - ST.UTCDiff + DT.Change - ST.Change, DateAndTime)) AS TimeZoneDifference,
ST.ClocksChangeDate, DT.ClocksChangeDate
FROM SampleData SD
CROSS APPLY (SELECT TOP 1 *
FROM DaylightSaving sq
WHERE sq.ClocksChangeDate <= SD.DateAndTime
AND Timezone = SD.Timezone
ORDER BY sq.ClocksChangeDate DESC) ST
CROSS APPLY (SELECT TOP 1 *
FROM DaylightSaving sq
WHERE sq.ClocksChangeDate <= DATEADD(HOUR, sq.UTCDiff - ST.UTCDiff,SD.DateAndTime)
AND sq.Timezone = 'GMT' --your destination Timezone
ORDER BY sq.ClocksChangeDate DESC) DT;
GO
DROP TABLE DaylightSaving;
DROP TABLE SampleData;
Edit: Note that that the difference between the 2 zones was hard coded (I have a DATEADD with a hard coded value of 5), as this is an example. You will need to extrapolate this for your own system, but as we have no concept of what that looks like, you'll need to do that work.
Edit 2:Felt like adding the timezone diffs in. Only hard coded value now is the value of the local timezone ('GMT' in this case in the second CROSS APPLY)
As several people have said, datetime has no concept of timezones. If this is paramount to your data, then don't use datetime, use datetimeoffset. That stores the UTC +/- value as part of the value; making things like this trivial.