Do you not have an appointment card with the date and time on it? I never leave without one, even if I put it in my calendar. That way if I screw up putting it in my own calendar, or somehow they screw up in putting it in their system (it happens), then I have the written date and time to show them or double-check with.

If you aren't expecting a med change, just reschedule if necessary so that you don't miss work. If their is no med change involved, you probably don't need monthly appointments anyway.