Setting the TOD clock

MVS™ bases its decisions and activities on the assumption that time is progressing forward at a constant rate. The instrument used to mark time in an MVS system is the time of day (TOD) clock. As it operates, MVS obtains time stamps from the TOD clock. MVS uses these time stamps to:

Identify the sequence of events in the system
Determine the duration of an activity
Record the time on online reports or printed output
Record the time in online logs used for recovery purposes.

To ensure that MVS makes time-related decisions as you intend, IBM® recommends that you do not reset your TOD clock, for example, to switch to or from Daylight Savings Time. Instead, set the TOD clock to a standard time origin, such as Greenwich Mean Time (GMT), and use the TIMEZONE statement of the CLOCKxx parmlib member to adjust for local time. (The TIMEZONE statement allows you to specify how many hours east or west of Greenwich you are.) To adjust local time, change the offset value for the TIMEZONE statement. This will not disturb MVS's assumption that time is progressing forward and will allow time stamps on printed output and displays to match local time.