SQL Function help- Daylight Savings calculation - sql

Question

All,
I wrote a function that essentially takes a timestamp and a shopper ID, and based on that shoppers zipcode work out the UTC offset and correct the time.
The problem I am having is: it's sloooow!
Can anyone see an easy way to speed it up ?
CREATE FUNCTION TimeModifier
(
-- Add the parameters for the function here
#InputDate datetime,#shopperid int
)
RETURNS datetime
AS
BEGIN
--Declare #shopperid int
--set #shopperid=25
-- Declare the return variable here
-- Declare the return variable here
DECLARE #Result datetime
Declare #zip nvarchar(10)
Declare #TimeofYear int
declare #Year int
declare #Winter int
declare #Summer int
select #year=datepart(yyyy,#InputDate)
--If 0 then its outside of the summer hours.
SELECT #timeofyear=count(*) FROM [d].[dbo].[DST-Dates] where #inputdate>=startdate and #inputdate<=enddate
select #zip=zip from d..shopper where shopperid=#shopperid
--Gets the UTC offset for winter and summer
select #winter=winter,#summer=summer FROM [MMD_Feed].[dbo].[ZipCodeZones] where zip=#zip
if(#TimeofYear=0)--IE is it Winter
set #Result=DATEADD(HH,#winter, #Inputdate)
else--Use summer offset
set #Result=DATEADD(HH,#summer, #Inputdate)
--select #Result
-- Return the result of the function
RETURN #Result
END
GO
Thanks
~J

Answer 1

Here is a total shot in the dark for converting this to an iTVF.
CREATE FUNCTION TimeModifier
(
-- Add the parameters for the function here
#InputDate datetime
, #shopperid int
)
RETURNS TABLE WITH SCHEMABINDING
AS RETURN
select DATEADD(Hour, case when t.TimeOfYear = 0 then z.winter else z.summer end, #InputDate) as MyResult
from d.dbo.shopper s
join [MMD_Feed].[dbo].[ZipCodeZones] z on s.zip = z.zip
cross apply
(
SELECT count(*) as TimeOfYear
FROM [d].[dbo].[DST-Dates]
where #inputdate >= startdate
and #inputdate <= enddate
) t
where s.shopperid = #shopperid