Understanding "Date Last Modified" for "System Stored Procedures"
According to BOL, the modify_date from sys.all_objects shows:


  Date the object was last modified by using an ALTER statement. If the object is a table or a view, modify_date also changes when a clustered index on the table or view is created or modified.


However, if you have a maintenance plan that recompiles stored procedures, using sp_recompile, or have system stored procedures that run with the WITH RECOMPILE option, this will also cause the modify_date to update.

There was a Connect Item raised about this, but it was closed as 'Won't Fix'.

This could potentially explain your Date Last Modified issue.