Converting GMT to EST in SQL Server - sql

Question

I have a table which stores records for Daylight saving and non-Daylight savings
CREATE TABLE #tmp
(
ID int,
IntervalName nvarchar(100),
StartDate datetime,
EndDate Datetime,
offset numeric(5, 2)
)
INSERT INTO #tmp
VALUES (1, 'EDT', '2022-03-13 07:00:00.000', '2022-11-06 06:00:00.000',-4.00)
INSERT INTO #tmp
VALUES (2, 'EST', '2022-11-06 06:00:00.000', '2023-03-12 07:00:00.000', -5.00)
I have my transactional tables which have Date column in the GMT timezone.
ID DatetimeGMT DatetimeLocal
---------------------------------------------------------
1 2022-11-05 07:00:00.000 2022-11-05 03:00:00.000
2 2022-11-10 06:00:00.000 2023-11-10 01:00:00.000
Now my DatetimeLocal column is calculating the offset hours based on the DatetimeGMT column.
The row with ID = 1 falls under offset -4 and the row with ID = 2 falls under offset -5.
Is there any suggestion how we can achieve this?

Answer 1

If you have the offsets all stored in a table like that, then you just need to JOIN to find the correct one.
eg
drop table if exists #tmp
drop table if exists #tran
go
CREATE TABLE #tmp
(
ID int,
IntervalName nvarchar(100),
StartDate datetime,
EndDate Datetime,
offset numeric(5, 2)
)
INSERT INTO #tmp
VALUES (1, 'EDT', '2022-03-13 07:00:00.000', '2022-11-06 06:00:00.000',-4.00)
INSERT INTO #tmp
VALUES (2, 'EST', '2022-11-06 06:00:00.000', '2023-03-12 07:00:00.000', -5.00)
create table #tran(id int primary key, DateTimeUTC datetime)
insert into #tran(id,DateTimeUTC) values (1,'2022-11-05 07:00:00.000'),(2,'2022-11-10 06:00:00.000')
select t.id, t.DateTimeUTC, dateadd(hour,offset,t.DateTimeUTC) DateTimeLocal, tz.offset
from #tran t
join #tmp tz
on t.DateTimeUTC >= tz.StartDate
and t.DateTimeUTC < tz.EndDate
outputs
id DateTimeUTC DateTimeLocal offset
----------- ----------------------- ----------------------- ---------------------------------------
1 2022-11-05 07:00:00.000 2022-11-05 03:00:00.000 -4.00
2 2022-11-10 06:00:00.000 2022-11-10 01:00:00.000 -5.00
SQL Server 2016 and later have the offsets built-in, so you can write queries like
select t.id,
t.DateTimeUTC,
cast(t.DateTimeUTC at time zone 'UTC' at time zone 'EASTERN STANDARD TIME' as datetime) DateTimeLocal
from #tran t