It returns a String value representing the host environment's current time zone, which is either a String representing a UTC offset for which IsTimeZoneOffsetString returns *true*, or a String identifier accepted by GetNamedTimeZoneEpochNanoseconds and GetNamedTimeZoneOffsetNanoseconds.
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It returns a String _id_ representing the host environment's current time zone as either an offset time zone or as an available named time zone identifier. In the former case, IsTimeZoneOffsetString(_id_) returns *true* and ParseTimeZoneOffsetString(_id_) returns an integer evenly divisible by 6 × 1010 (i.e., any second and fractional second units in _id_ are 0). In the latter case, the result from AvailableNamedTimeZoneIdentifiers() contains a Time Zone Identifier Record _r_ such that _r_.[[Identifier]] is _id_, and _id_ is valid as the first argument to GetNamedTimeZoneEpochNanoseconds and GetNamedTimeZoneOffsetNanoseconds.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the DefaultTimeZone abstract operation as specified in the ECMA-402 specification.