T 3.71 Incorrect system time on z/OS systems

The system time (date and clock time) is an important variable for a whole series of applications and system programs on which the correct execution of numerous actions and the reliable preparation of results and data is dependent.

Due to incorrect date/time information, among other aspects the following security problems and resulting damage can occur:

• Applications that make decisions based on the current date provide incorrect results. The rework of entire days of production can be the consequence. This applies particularly for online applications and their transaction data. Corrections are often no longer possible if, for example, customers access the system online.
• The analysis of security incidents that takes into account time information can be made significantly more difficult or even produce incorrect results.
• Differing system times in interconnected systems are a problem if, for example, log data are to be used for a common evaluation.
• Applications that receive data from several individual systems and process the data as a function of the time stamp will produce corrupted results.

System time on z/OS systems

If z/OS systems are not operated in a parallel sysplex cluster, as a rule it is necessary for the operator to enter the system time manually during the IPL (Initial Program Load). In doing so, mistakes in the date or time can easily be made.

It is also possible to change the system time when the system is in operation. Here the risk of incorrect entries is even greater than during an IPL.

In the member clock00 the time zone or the difference in relation to Greenwich Mean Time (GMT) is set. An incorrect time zone setting will produce the same result as if the system time itself was set incorrectly.

Examples:

During operation the time setting in a z/OS system was to be corrected by 5 minutes. A typing error on the entry of the SET command resulted in a system time that was in the evening. Accordingly the job scheduler started the evening batch production during the day. Because the batch jobs required exclusive access to the application data bases, data entry online was no longer possible.