Technical data

Setting System Time
6.1 Setting Correct Time Zone Information on Your System
6.1.2 Understanding Time-Setting Concepts
Understanding some time concepts will help you see the importance of setting
correct time zone information on your system.
6.1.2.1 Coordinated Universal Time
Coordinated Universal Time (UTC) is similar in most respects to Greenwich
Mean Time (GMT). Under the UTC time standard, zero hours occurs when the
Greenwich Meridian is at midnight. Unlike local time, which can go backward
and forward depending on daylight saving time, UTC always increases.
Local times can be up to 12 hours behind Greenwich Mean Time or 13 hours
ahead of it.
Because UTC is independent of time zones, you can use UTC around the world;
for example, it is 2:00 UTC at the same moment in Paris as it is in Tokyo. You
can examine data that is time-stamped with UTC values in Paris and Tokyo
without performing complicated conversions to deal with local time zones.
6.1.2.2 Time Zones
Time zones for geographical areas that share the same local time also share the
same rule or rules for seasonal changes between standard time and daylight
saving time.
6.1.2.3 Daylight Saving Time and Standard Time
Typically, you make seasonal changes to the local system time (for example, for
daylight saving time and standard time). You usually adjust the local time one
hour forward or backward.
6.1.2.4 Time Differential Factor
One of the steps in setting the correct time on your system is to calculate a time
differential factor (TDF) for your time zone.
The TDF associates each local time zone with UTC; it is the difference between
your local system time and UTC. When your system time changes to reflect a
local time change between standard time and daylight saving time, the TDF must
also change to compensate for the new local system time. The TDF changes in
the same direction as the local time; that is, if you add an hour to the local time,
you must also add an hour to the TDF. Note, however, that the UTC does not
change.
The TDF value is expressed in signed (+ or -) hh:mm format. The Americas have
negative TDFs, while Europe, Africa, Asia, and Australia have positive TDFs.
OpenVMS procedures calculate the normal TDFs for standard and daylight
saving time (if any) for your time zone. These are presented as the default for
your TDF setting. Compaq recommends that you accept this default.
You can also use the map in Figure 6–1 to determine the TDF for your time zone.
If you prefer, you can use the tables in Appendix B in the OpenVMS System
Manager’s Manual to determine the standard or daylight saving time TDF for
your time zone.
To use the map to determine the TDF of your time zone, follow these steps:
1. Find your location on the map and notice the time zone band at that location.
2. Follow the time zone band to the top of the map and note the TDF that
corresponds to your time zone. For example, the TDF for California is –8:00;
the TDF for Italy is +1:00.
6–2 Setting System Time