When running a job's JCL, you may often use JMF's OSDDATE or TODDATE variables. This explains the differences on which actual date will be substituted.
Using the OSDATE variables for example, the variable used as OSDAY3. The day the event is placed onto the checkpoint will be used to resolve the JMF variable. In this case, it doesn't matter what day the job actually processes on the system.
Using the TODDATE variables as an example OSDAY3. The day the event is actually run will be used to resolve the JMF variable regardless of when it is placed on the checkpoint.