Cache routines: Date and Time Saved vs. Date and Time Modified

My objective is to find routine's Date/Time Saved someplace inside Cache. The routine's Date/Time Modified as returned by various queries like %Library.RoutineIndex and as seen on the File Open screen is actually Date/Time Compiled last, not Date/Time Saved last. Does anyone know if Date/Time Saved is created when the Save icon is pressed or the routine is imported? Such a timestamp should be immune to subsequent compilations with no changes.

I am also curious about the internal Cache mechanism that marks the routine's tab with + which means saved but not compiled yet. Does it compare some invisible timestamp Saved with the timestamp Compiled?