11.3 DEC C RTL Date/Time Computations-UTC and Local Time

Universal Coordinated Time (UTC) is an international standard for measuring time of day. Under the UTC time standard, zero hours occurs when the Greenwich Meridian is at midnight. UTC has the advantage of always increasing, unlike local time, which can go backwards/forwards depending on daylight saving time.

Also, UTC has two additional components:

• A measure of inaccuracy (optional)

• A time-differential factor, which is an offset applied to UTC to derive local time.

  The time-differential factor associates each local time zone with UTC; the time differential factor is applied to UTC to derive local time. (Local times can vary up to -12 hours West of the Greenwich Meridian and +13 hours East of it).

For the DEC C RTL time support to work correctly on OpenVMS Version 7.0 and higher, the following must be in place:

• Your OpenVMS system must be correctly configured to use a valid OpenVMS TDF. Make sure this is set correctly by checking the value of the SYS$TIMEZONE_DIFFERENTIAL logical. This logical should contain the time difference added to UTC to arrive at your local time.

• Your OpenVMS installation must correctly set the local time zone that describes the location that you want to be your default local time zone. In general, this is the local time zone in which your system is running.

For more information, see the section on setting up your system to compensate for different time zones in your OpenVMS System Manager's Manual: Essentials.

The DEC C RTL uses local time-zone conversion rules to compute local time from UTC, as follows:

1. The DEC C RTL internally computes time in terms of UTC.

2. The DEC C RTL then uses time-zone conversion rules to compute a time-differential factor to apply to UTC to derive local time. See the tzset function in the reference section of this manual for more information on the time-zone conversion rules.

By default, the time-zone conversion rules used for computing local time from UTC are specified in time-zone files defined by the SYS$LOCALTIME and SYS$POSIXRULES system logicals. These logicals are set during an OpenVMS installation to point to time-zone files that represent the system's best approximation to local wall-clock time:

• SYS$LOCALTIME defines the time-zone file containing the default conversion rules used by the DEC C RTL to compute local time.

• SYS$POSIXRULES defines the time-zone file that specifies the default rules to be applied to POSIX-style time zones that do not specify when to change to summer time and back.

SYS$POSIXRULES can be the same as SYS$LOCALTIME. See the tzset function for more information.