You hit a common error. Don't use DateTime, but UTCDateTime.

http://msdn.microsoft.com/en-us/library/ms973825.aspx

>There is a function Date.getTimezoneOffset() which returns the number of offset minutes from GMT, which takes care automatically of the Daylight Saving Time. However, if I would like to know if the browser is being executed from a PC where the Daylight Saving Time is in effect, would there be a way to know it?