Support for Daylight Saving Time
Oracle automatically determines whether Daylight Saving Time is in effect for a specified time zone and returns the corresponding local time. The datetime value is usually sufficient for Oracle to determine whether Daylight Saving Time is in effect for a specified time zone.
The periods when Daylight Saving Time begins or ends are boundary cases. For example, in the Eastern region of the United States, the time changes from 01:59:59 a.m. to 3:00:00 a.m. when Daylight Saving Time goes into effect. The interval between 02:00:00 and 02:59:59 a.m. does not exist. Values in that interval are invalid. When Daylight Saving Time ends, the time changes from 02:00:00 a.m. to 01:00:01 a.m. The interval between 01:00:01 and 02:00:00 a.m. is repeated. Values from that interval are ambiguous because they occur twice.
To resolve these boundary cases, Oracle uses the TZR and TZD format elements. TZR represents the time zone region in datetime input strings. Examples are 'Australia/North', 'UTC', and 'Singapore'. TZD represents an abbreviated form of the time zone region with Daylight Saving Time information. Examples are 'PST' for US/Pacific standard time and 'PDT' for US/Pacific daylight time. To see a list of valid values for the TZR and TZD format elements, query the TZNAME and TZABBREV columns of the V$TIMEZONE_NAMES dynamic performance view.
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