The date of an appointment might change, but the date itself does not. Dates are values that refer to a fixed period of time; the meaning of "March 7th 2013" won't change if I move an appointment.
The correct way to handle an appointment is to have a reference to an immutable date. When you want to change the date the appointment is on, you change the reference to a different date, not the date itself.
The correct way to handle an appointment is to have a reference to an immutable date. When you want to change the date the appointment is on, you change the reference to a different date, not the date itself.