Daylight saving time

Where it is used, Daylight Saving Time (DST) maximizes daylight hours during normal waking hours. Many time zones, but not all, use this twice-yearly event to adjust the local time.

The 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. The 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. Since DST policies are set by national and regional governments, these rules vary widely from zone to zone. Also, DST policies are subject to change for this same reason—as in the 2007 change for all U.S. time zones that observe DST.

For example, in the 2007 the United States changed the DST start time to the first Sunday on or after the 8th of March (from first Sunday on or after the 1st of April for 2006 and prior years). It changed the DST end time to the first Sunday on or after the 1st of November (from the last Sunday in October for 2006 and prior years).

Note: A change in DST rules for a time zone can cause issues in Niagara when displaying historical data (histories and alarm records), particularly when applying new (current) DST rules to records collected using prior (old) DST rules.

Java comes with a time zone database that has an historical perspective. This means that it stores a history of changes for applicable time zones. Thus, multiple definitions for a given time zone may exist including past definitions as well as its current definition. This facilitates the display of a station’s time-stamped data (histories and alarms) collected in time zones under prior rules. These are usually DST-related rules. These historical definitions ensure that records display with the original time the data were collected.

In a Niagara 4 JACE controller, the Java source time zone database is historically accurate only back to the year 2010. Any pre-2010 historical data display using 2010 rules. This improves Java heap usage on these platforms.

On a Windows platform used by a Supervisor PC the Java source time zone database extends back to 1995.

To view the time zone DST rules for the station currently running on a host, select Tools and click Time Zone Database Tool. Getting Started with Niagara provides more information about this tool.

For former AX users, Niagara 4 requires no separately maintained timezones.jar distributed in the builds, nor associated entries in a platform’s system.properties file. Instead, time zones are directly sourced from the Java VM (virtual machine) in the host platform. This means there is no requirement to update time zone definitions independently from Java updates that may be included in Niagara 4 updates.