Is there an easy way to let the SF handle timezones for one server?

I have developed 2 SQL functions for this:

create function utc_to_local_datetime
(
 @utc_date_time date_time ,
 @time_zone_id time_zone 
)
returns date_time
as
begin


 DECLARE @offset_minutes AS INT

 SET @time_zone_id = COALESCE(@time_zone_id, dbo.get_user_setting('time_zone_id', NULL), 'Central European Standard Time')

 SET @offset_minutes = (SELECT CAST((LEFT(offset, 1) + '1') AS INT) * DATEDIFF(MINUTE, '1/1/1900', CAST(right(offset, 5) AS DATETIME)) FROM (SELECT offset = RIGHT(CONVERT(DATETIME, @utc_date_time) AT TIME ZONE @time_zone_id, 6)) x)

 RETURN DATEADD(MINUTE, @offset_minutes, @utc_date_time)


end
go

grant execute on utc_to_local_datetime to public
go

create function local_to_utc_datetime
(
 @date_time date_time ,
 @time_zone_id time_zone 
)
returns date_time
as
begin

 DECLARE @raw_offset_minutes AS INT

 SET @raw_offset_minutes = (SELECT CAST((LEFT(offset, 1) + '1') AS INT) * DATEDIFF(MINUTE, '1/1/1900', CAST(right(offset, 5) AS DATETIME)) FROM (SELECT offset = RIGHT(CONVERT(DATETIME, @date_time) AT TIME ZONE @time_zone_id, 6)) x)

 RETURN DATEADD(MINUTE, -1 * @raw_offset_minutes, @date_time)

 -- OTHER SOLUTION (NOT WORKING WITH NEGATIVE OFFSET)
 -- RETURN CONVERT(DATETIME, @date_time) - CONVERT(CONVERT(TIME, RIGHT(CONVERT(DATETIME, @date_time) AT TIME ZONE @time_zone_id, 5)))


end
go

They work fine, but it is still hard to let them work for al scenarios. For example a user in Japan, who wants to schedule a task in Brazil 6 months in advance.

A local time control would be very welcome. Solving the problem with SQL is possible, but sometimes has serious impact on performance.