This is explained in tutorial 8: Google Code Archive - Long-term storage for Google Code Project Hosting.
When you jump from your toplevel main menu to the date/time entry you need to call a special procedure (prepare_user_input()) which reads data from external sources and puts them into variables which are shared with m2tklib.
If you would use a list of M2_BUTTON elements as toplevel menu, then the callback procedure for the M2_BUTTON should
(A) read the data from RTC and (B) refer to the date entry menu by using setRoot() procedure.
Tutorial 8 uses M2_STRLIST instead of M2_BUTTON, but the idea is the same.
Oliver