$TimeZone

gives the current time zone to assume for dates and times.

Details

The time zone gives the number of hours that must be added to Greenwich Mean Time (GMT) to obtain local time.
US Eastern Standard Time (EST) corresponds to time zone -5.
Daylight saving time corrections must be included in the time zone, so US Eastern Daylight Time (EDT) corresponds to time zone -4.
$TimeZone defaults to $SystemTimeZone.
$TimeZone=z resets the current time zone for purposes of functions like DateString, DateList, and AbsoluteTime.
Settings for $TimeZone may be an Integer, Real, or time zone String.

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Show the number of hours that must be added to GMT to obtain local time:

$TimeZone is set to the current GMT offset when the Wolfram System is initialized:

If the session persists across a daylight time transition, the offset will not change automatically:

In such cases, $TimeZone may be reset manually, or a named time zone may be used to account for the change:

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