DATETIMEOFFSET is not daylight savings time-aware. If you want to calculate the offset for the current time, you can use this:
SELECT DATEDIFF(hour,GETDATE(),GETUTCDATE())In US Central Time Zone, it will return 5 hours during daylight savings time period and 6 during standard time period.
If you are trying to calculate historical data, you will need something like they have in the link that Visakh posted earlier. However, the code in there is incorrect for dates earlier than 2007. An act of Congress messed it up for dates earlier than 2007. (Yes, I realize that last sentence is stating the obvious about US Congress).