Adjusting Time Zone And Daylight Saving In SQL

Hi,

I have this simple SQL query which SELECTs the fields according to the criteria in WHERE clause.  SELECT callingPartyNumber, originalCalledPartyNumber, finalCalledPartyNumber,
dateadd(ss, (dateTimeConnect + (60 * 60 * -5))+3600 , '01-01-1970 00:00:00') AS dateTimeConnect,
dateadd(ss, (dateTimeDisconnect + (60 * 60 * -5))+3600, '01-01-1970 00:00:00') AS dateTimeDisconnect,
CONVERT(char(8), DATEADD(second, duration, '0:00:00'), 108) AS duration
FROM Calls
WHERE
(callingPartyNumber = ISNULL(@callingPartyNumber, callingPartyNumber)) AND
(originalCalledPartyNumber = ISNULL(@originalCalledPartyNumber, originalCalledPartyNumber)) AND
(finalCalledPartyNumber = ISNULL(@finalCalledPartyNumber, finalCalledPartyNumber)) AND
(duration >= @theDuration) AND
((datetimeConnect - 14400) >= ISNULL(convert(bigint,
datediff(ss, '01-01-1970 00:00:00', @dateTimeConnect)), datetimeConnect)) AND
((dateTimeDisconnect - 14400) <= ISNULL(convert(bigint,
datediff(ss, '01-01-1970 00:00:00', @dateTimeDisconnect)), dateTimeDisconnect))

 If you notice, in the SELECT, I add 3600 (1 hour) to adjust the time according to the current DayLight saving setting. Similarly, while comparing in WHERE clause, I subtract 14400 seconds (4 hours) to adjust the comparison according to EST (-5 GMT, but doing -4 GMT since daylight saving) and daylight saving.

Also, dateTimeConnect and dataTimeDisconnect fields save time in UTC seconds.

Now this application may be used by clients in different timezones. The application may be hosted on their server which would mean a different time zone. What would be the best way to make this time zone and daylight adjustment more dynamic instead of hardcoding the values.