W2KSG: Time Zones
The WMI section of the scripting guide gives a very simple method of determining the time zone on your machine
Get-WmiObject Win32_TimeZone | Format-List
Bias : 0
Caption : (GMT) Greenwich Mean Time : Dublin, Edinburgh, Lisbon, London
Bias returns the offset from GMT and the caption returns the time zone. Information on daylight saving settings is also available - try get-member on it!