when adding the paypal standard button, the third screen gives the a filed to define an option for the add to cart, this will allow you to add the date option. name it "Appointment Date"
this will add the following 2 fields:
date:<input type="hidden" name="on0" value="Appointment Date" />
<input type="text" name="os0" value="" />
select the os0 text field, then add a date picker using data Bridge.
then add to more fields: a hidden file named on1, set the value to Appointment Time" and a select list named os1, populate it with the available times