Effect of daylight saving time changes
Many countries operate a policy of adjusting clocks by one hour at the beginning and end of summer to effect what is commonly referred to as daylight saving.
These time changes are also applied to the local time held in computers. Generally, most hardware (TOD) clocks are set to Greenwich Mean Time (GMT), with an offset value to indicate local time. It is this offset value that is adjusted when making daylight saving time changes, while the hardware clock remains unaltered.
Adjusting local time automatically
Specify the system initialization parameter AUTORESETTIME=IMMEDIATE to synchronize the CICS® time with the z/OS® time immediately whenever you alter the system date or time-of-day in the MVS™ TOD clock while a CICS region is running. AUTORESETTIME=IMMEDIATE makes CICS issue a PERFORM RESET command to synchronize the CICS time-of-day with the system time-of-day if, at the next task attach, the CICS time-of-day differs from the system time-of-day. The default setting for AUTORESETTIME is IMMEDIATE. If you specify an alternative setting, ensure that you have a process in place to guarantee that a manual CEMT PERFORM RESET or EXEC CICS PERFORM RESETTIME command is issued immediately after altering the MVS TOD clock.
Adjusting local time manually
When setting clocks forward or back an hour to adjust for Summer and Winter time while a CICS region is running, use the CEMT PERFORM RESET or EXEC CICS PERFORM RESETTIME command to ensure that CICS immediately resynchronizes its local time with that of the MVS TOD clock.
CICS obtains and stores the local time offset at start up, and when the CEMT PERFORM RESET command executes. Use the CEMT PERFORM RESET command immediately whenever you change the system date or time-of-day while CICS is running, to ensure that the correct local time is used by all CICS functions, including the API. Whenever an application program issues an EXEC CICS ASKTIME command, CICS obtains the current time from the MVS TOD clock, and modifies this by the stored local time difference. CICS then updates the EIBTIME field in the exec interface block with the local time.