System logger uses the system clock GMT value as an authorization key when writing to the coupling facility on behalf of the log stream.

If you change the GMT, specifically, turn the clock back, system logger will not be able to write to the log stream until the new GMT is greater than the old GMT. When changing the GMT, you should delete all of the log streams and then redefine them, especially if the GMT is moved backwards.

Changing the GMT affects both coupling facility and DASD-only log streams.

Example 1:A coordinated timing network (CTN) is a collection of servers and Coupling Facilities that are time synchronized to a time value called coordinated server time. A mixed-coordinated timing network (CTN) requires a Sysplex Timer and allows for both an external time reference (ETR) network and a server time protocol (STP).

There are circumstances when the TOD clock might drift enough for system logger to detect that a system on an ETR-timing CPC may be less advanced than one on the STP-Timing CPC. The drift can occur based on how long the stratum 2 server resides in STP-timing mode during reverse migration from Mixed-CTN to an ETR-only network on z/OS® images on the CPC for which the ETR ports are enabled.

The TOD clock drift might result in system logger issuing an ABEND01C5, rsn00040003 and a dump is taken to record the condition. System logger continues operating without any other problems, but log data cannot be written into the log stream from this system until the condition is cleared. See z/OS MVS System Codes for System Action and System Programmer Response for this condition.

Example 2: Log stream “IXGLOGR.TEST” is an active log stream and contains data written by an application using system logger services. The system is IPLed. The last block written, before the IPL, was written at 13:00:00 Mar 1, 2000. This date and time is the authorization key used by system logger. During the IPL, the GMT is changed to 13:00:00 Mar 1, 1998. When system logger attempts to write the next block to IXGLOGR.TEST, it will use the new GMT. System logger will detect the difference in the GMT values and abend.