The following is from the CA 7 r12.0 release notes:
For multi volume data sets, previous releases used the date and time of the last volume created. In Version 12.0, the date and time of the first volume that is created is used. For data sets that are opened or closed multiple times within a job step, previous releases used the date and time of the last creation. In Version 12.0, the date and time of the first creation is used.
question; Will this enhancement affect external dataset triggering or dataset requirement posting?
No, this enhancement will not affect external dataset triggered jobs or jobs waiting on dataset requirements. It only affect the date displayed when the LDSN or LCTLG command is issued. Nothing to be concerned about.