There is nothing in the dictionary definition of the dialog that records any information about when the dialog was last executed.
You can only extract this from time-based reports such as SREPORTs, Performance Monitor reports, or archive journals.
But obviously this will only help for as far back as what archive logs and journals you have kept.
ADS related SREPORTS are documented at ADS Dialog Statistics.
SREPORT 019 might be useful.
Performance Monitor Reports
PMARPT04 or PMARPT03 will report when dialogs were executed.
If the dialogs update database records or if you have JOURNAL RETRIEVAL set in the Sysgen SYSTEM Statement, then the fact that a dialog was executed will be in the journals.
JREPORT 004 will report this.