TimeZone[ ] gives the time zone set for your computer system.
The time zone gives the number of hours which must be added to Greenwich mean time (GMT) to obtain local time.
U.S. eastern standard time (EST) corresponds to time zone .
Daylight saving time corrections must be included in the time zone, so U.S. eastern daylight time (EDT) corresponds to time zone .
See The Mathematica Book: Section 2.13.3.
See also: Date, AbsoluteTime.
Related package: Miscellaneous`CityData`.