Adjusting local time in a sysplex

The following actions are important whether or not your installation has decided to operate using the TOD clock set to the local time of the installation, or GMT time. It is of the utmost importance that the TOD clock value not be changed on any of the processors in the sysplex once a value has been established for a running sysplex. The TOD clock on all processors must be using the identical time, or be synchronized to the same time, because that is the value used by software. The value known as local time, which is computed as an offset from the TOD clock value, may be changed at any time. It is not the value used by time-critical software to perform tasks. To adjust local time without resetting the TOD clock in the processor, you can:

Reset the time offset in the Sysplex Timer
Change the local time offset in the CLOCKxx member of parmlib
Issue the SET CLOCK command
Adjust time zone settings with STP, using the HMC

The method you choose to adjust local time depends on your sysplex configuration and on the options specified on the ETRMODE, ETRZONE, and SIMETRID statements of the active CLOCKxx member.

Sysplex includes a Sysplex Timer
- If the time-zone offset is taken from the Sysplex Timer (ETRMODE YES and ETRZONE YES), change the time offset from the Sysplex Timer console. See Planning for the 9037 Sysplex Timer (GA23-0365) for additional details.
  There is no need to reIPL the system to cause the local time adjustment to take effect.
- If the time-zone offset is taken from the CLOCKxx member (ETRMODE YES and ETRZONE NO) do either of the following:
  - Issue the SET CLOCK command to change the local time offset and change the local time offset value in the CLOCKxx member to reflect the new offset. This will preserve the change for ensuing IPLs.
  - Change the offset value in CLOCKxx and re-IPL.
Sysplex does not include a Sysplex Timer
- In a single system sysplex, do either of the following:
  - Issue the SET CLOCK command to change the local time offset, and change the local time offset value in the CLOCKxx member to reflect the new offset. This will preserve the change for ensuing IPLs.
  - Change the offset value in CLOCKxx and re-IPL.
- In a multisystem sysplex on one processor (either guest MVS™ systems running under a host VM system or in logical partitions (LPARs) in a PR/SM™ environment), do either of the following:
  - Update the CLOCKxx member for each guest MVS system with the correct time-zone value and reIPL each system.
  - Issue the SET CLOCK command to change the local time.
Systems that are not attached to the Sysplex Timer must be reIPLed in order to begin using the new time-zone value in CLOCKxx.
For more information about STPMODE settings, see Server Time Protocol Planning Guide, which is available at the following URL:
```
http://www.redbooks.ibm.com/redbooks/pdfs/sg247280.pdf
```