You haven't stated the issue or problem? What you seem to want should not be too hard though.
Consider calculating this date rather than storing it. If based on another field e.g. WaitPeriod (where the wait period in days could be any number depending on factors such as the patient, treatment, regulations, etc) you can add customized periods to the last visit date and compare to "today". Aside from a date that you enter, there isn't much difference in what you'll be able to get out of either method. That is, either way you can report on x days before or past due. I consider knowing before something is overdue to often be of more value than knowing only when it's overdue.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.