You don't want to use a function against the END_DATETIME column if you can avoid it, since any function will prevent SQL from using the full seek capabilities against that column in any index(es) in which it might appear.

I think the code below will do SELECT the same rows as your original query, but adjust the code if/as needed to match you need:

SELECT *

FROM dbo.tablename

WHERE

END_DATETIME >= DATEADD(HOUR, -@tzoffset, @StartDate) AND

END_DATETIME <= DATEADD(HOUR, -@tzoffset, @EndDate)