A time zone is a geographic area that has adopted the same standard time, usually referred to as the local time.
Most adjacent time zones are one hour apart. By convention, all time zones compute their local time as an offset from GMT/UTC. GMT (Greenwich Mean Time) is an historical term, originally referring to mean solar time at the Royal Greenwich Observatory in Britain. GMT has been replaced by UTC (Coordinated Universal Time), which is based on atomic clocks. For all Sybase Event Stream Processor purposes, GMT and UTC are equivalent. Due to political and geographical practicalities, time zone characteristics may change over time. For example, the start date and end date of daylight saving time may change, or new time zones may be introduced in newly created countries.
Internally, Event Stream Processor always stores date and time type information as a number of seconds, milliseconds, or microseconds since midnight January 1, 1970 UTC, depending on the datatype. If a time zone designator is not used, UTC time is applied.
Daylight saving time is considered if the time zone uses daylight saving time and if the specified timestamp is in the time period covered by daylight savings time. The starting and ending dates for daylight saving time are stored in a C++ library.
If the user specifies a particular time zone, and if that time zone uses daylight saving time, Event Stream Processor takes these dates into account to adjust the date and time datatype. For example, since Pacific Standard Time (PST) is in daylight saving time setting, the engine adjusts the timestamp accordingly:
to_timestamp('2002-06-18 13:52:00.123456 PST','YYYY-MM-DD HH24:MI:SS.ff TZD')
During the transition to and from daylight saving time, certain times do not exist. For example, in the US, during the transition from standard time to daylight savings time, the clock changes from 01:59 to 03:00; therefore 02:00 does not exist. Conversely, during the transition from daylight saving time to standard time, 01:00 to 01:59 appears twice during one night because the time changes from 2:00 to 1:00 when daylight saving time ends.
However, since there may be incoming data input during these undefined times, the engine must deal with them in some manner. During the transition to daylight savings time, Event Stream Processor interprets 02:59 PST as 01:59 PST. When transitioning back to standard time, Event Stream Processor interprets 02:00 PDT as 01:00 PST.