DST

Daylight Saving Time (DST) is used as a means of maximizing daylight hours during normal waking hours, and is used by many (but no means all) time zones. DST is a twice-yearly event acting upon local time, as follows:

Start of DST adds an offset (typically 1 hour) to local time. During this period of the year, local time may be called “daylight time.”
End of DST removes the DST offset from local time. During this period of the year, local time may be called “standard time.”

Any time zone using DST has specific rules that define the exact days and times when DST starts and ends. These rules vary widely from zone to zone, since DST policies are set by national and regional governments. Also, DST policies are subject to change for this same reason—as in the recent 2007 change for all U.S. time zones that observe DST.

In the 2007 U.S. DST changes, the DST start time was changed to “first Sunday on or after the 8th in March” (from “first Sunday on or after the 1st in April” for 2006 and prior years). The DST end time was changed to “first Sunday on or after the 1st in November” (from “last Sunday in October” for 2006 and prior years).

A change in DST rules for a time zone can cause issues in NiagaraAX when displaying historical data (histories and alarm records), particularly when applying new (current) DST rules to records collected using prior (old) DST rules. These issues have been overcome using a “historical timezones” mechanism. For more details, see About the historical time zone database.