System time can be initialized with any value. To do this, the system start must be performed in DIALOG mode with the IPL option UNLOCK. By responding to the output messages the operator can specify any time. The SVP time remains unaffected by this.
Under VM2000, it is possible to run any VM with a fictitious system time and correspondingly offset UTC time. However, this guest system must always be started in DIALOG mode with the IPL option UNLOCK.
No synchronization is then performed with any external timer which may be present when the manually set time is more than 15 minutes different from the SVP time. The system time is clocked with the hardware TODR’s quality.
Since the modified system time is also used for time stamps and the system catalog, then, for example, if the system time has been put forward and the pubset is then reused at the ACTUAL time, file access may be rejected because the EXPIRATION date has not yet arrived. Application-specific data with time stamps may also be unpredictably inconsistent after such dual manipulations and may result in undefined program behavior (in particular in the case of databases).
The permanent operation of a system with a foreign time zone represents a special case. Conversion to another time zone in the monitor system or in the native system works only if the new time zone is set in the SVP resp. carrier system and startup is then performed with IMPL.
Otherwise (i.e. in particular in the case of a foreign time zone for a guest system) the system may not use the time received from the SVP at startup via Store Real Clock as the system time. The correct time for the required time zone can only be obtained by being input manually. To permit this, every startup of this system must be implemented as a dialog startup with the UNLOCK option.
The following must also be taken into account:
If the GTIME parameter file only contains information on the required time zone, the CTIME macro cannot carry out any conversions for foreign time zones during ongoing operation.
If the GTIME parameter file contains information on multiple time zones, it is not possible to recognize which of these is the correct one after the system time has been entered manually. The time initialization routine then engages in a message dialog with the operator which enables a zone to be entered but not a difference between summer time/winter time or a changeover points for this zone.
The zone entered is stored in the SVL , but effective operation is not possible with such a system. Only at the next startup – naturally in DIALOG UNLOCK mode – is the zone stored in the SVL used in place of the SVP zone, thus permitting evaluation of the converted parameter file.