$TimeZone

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

Details

	n	number of hours added to GMT
	"zone"	named time zone
	Entity["TimeZone", "zone"]	time zone specified by the "TimeZone" entity
	loc	time zone associated with the given location
	Automatic	automatically determine the time zone
	None	specify no time zone information

For numeric values n, the value represents the number of hours that must be added to Greenwich Mean Time (GMT) to obtain local time.
For string specifications "zone", the result will be returned in the associated named time zone.
Example time zone strings include "America/Chicago", "Europe/London", "Asia/Tokyo" and "Zulu".
A list of available time zone entities can be found using EntityList["TimeZone"].
For location specifications loc, the time zone associated with the specified location Entity or GeoPosition will be used.
$TimeZone defaults to $SystemTimeZone.
$TimeZone=z resets the current time zone for purposes of functions like DateString, DateList, and AbsoluteTime.

open allclose all

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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