Consequences of resetting the TOD clock in a sysplex

Never change the TOD clock that the sysplex is using, once it is set. Doing so can lead to a sysplex-wide IPL, or having working systems within the sysplex at risk of being partitioned out of the sysplex (that is, placed in a non-restartable wait state). Whether you choose to set the TOD clock to local time or to GMT time, local time changes can be accomplished using one of the techniques discussed in Adjusting local time in a sysplex. You should be aware of the possible consequences to XCF and the multisystem applications using XCF services when you set the TOD clock backward or forward:

Setting the TOD Clock Backward
As it monitors system status, MVS™ records, in the sysplex couple data set, the time stamps obtained from the TOD clock. If you set the TOD clock back, then time stamps obtained from the clock may duplicate time stamps already recorded in the sysplex couple data set. For example, if the TOD clock is reset to 2:00 AM at 3:00 AM, then all timings between 2:00 and 3:00 will occur again. When a system attempts to IPL into the sysplex, MVS compares time stamps from the TOD clock with time stamps recorded in the sysplex couple data set. If time stamps in the couple data set are later than those specified by the incoming system, MVS determines this is an error and does not allow the system to join the sysplex. The system is not allowed to IPL into the sysplex until the time stamp from the TOD clock is greater than time stamps recorded in the sysplex couple data set. In the example described, this condition would persist for one hour.

Furthermore, it is common practice for many MVS applications to record time stamps on external media. For example, time stamps might be written to a data set containing a database log. These logs could be used by several processes, such as database recovery, which depend critically on progressively increasing time stamps to preserve the ordering of events. Setting the TOD clock backward could make the log useless for database recovery.
Setting the TOD Clock Forward
As part of its status monitoring, MVS periodically checks the status of each system in the sysplex. To determine system failure, MVS uses the failure detection interval value specified in the COUPLExx parmlib member. If you set the TOD clock ahead, then the difference between a time stamp previously recorded in the couple data set and a time stamp obtained from the reset TOD clock could exceed the specified failure detection interval. In this case, MVS would initiate a “status update missing” condition, which in turn would cause the other system(s) to be removed from the sysplex (that is, placed in a non-restartable wait state). This would leave only the system with the future TOD clock in the system or sysplex.