See also the chapter “System time administration”.
There are three sources for determining the date and time during system initialization:
SVP clock: returns local time
On SUs /390 the SVP clock is synchronized by the Management Unit.
On SUs x86 the SVP clock is emulated by X2000. The SVP time corresponds to the time of the carrier system.
CPU clock (TODR): provides the local time with a correction value stored on the home pubset. This clock continues to run even if the CPU stops, but not if the power supply is interrupted.
On SUs /390 the TODR is an autonomous clock.
On SUs x86 the TODR is emulated by X2000.
Operator
The currently valid time is displayed for the operator at the console. In the event of an error or in DIALOG mode (using the UNLOCK option), the operator receives the appropriate messages at the console and is requested to confirm or to make corrections.
The local time (LT) is the statutory time at the location at which BS2000 is installed. Because of statutory changes (summer/winter time) it does not run continuously once set; instead, it requires a positive or negative correction at the changeover times. This correction is made automatically with the aid of the GTIME parameter file in which the correction value and the changeover times are stored (see "System time control (GTIME)").
The system time is based on the local time. BS2000 cannot run without the system time.
Systems support uses the parameters for time zone, time increments, summer and winter time and the changeover data to define the basis for calculating the universal world time UTC from the local time LTT and for summer/winter time changes of local time during system execution.
In this way, the system and the users can use the GTIME function to access both a local time reference system and a time reference system valid for the entire system.
Determining the time automatically for manual startup
The time is determined automatically when the following conditions are met:
The IPL option UNLOCK is not set, i.e. system initialization was carried out in FAST or AUTOMATIC mode or without the IPL option UNLOCK in DIALOG mode.
The time of the SVP clock is valid.
The time of the SVP clock has the correct format (e.g. not 31.2.14).
The time of the SVP clock is greater than the time of the last session and the positive increment compared with the last session is not greater than 6 days (exception: in AUTOMATIC mode, the positive increment may be any size).
If a shutdown is performed immediately before switching from daylight savings time to normal time and the startup immediately thereafter, system initialization starts with a time less than the last session. This is only possible if the UNLOCK option was set in DIALOG mode (see "DIALOG startup").
Negative time increments outside these changeover periods are permissible only by agreement with systems support. There is a risk of introducing inconsistencies into the data resources.
Influencing the time definition in DIALOG startup
If DIALOG mode is used and the UNLOCK option set, the operator is prompted for confirmation even if the value of the SVP clock is valid. If the time (and the date) of the SVP clock is not confirmed, the operator must enter the date and time himself. It is possible to specify large positive and negative time increments in relation to the last session. Once the time has been specified in this way and confirmed by logging, it is used as the valid system time for the next session.