Displaying items in a nested listview, based on the other listview
The microflow DS_Rooster_bij_persoon_gesorteerd_op_datum should have a parameter of Rooster_Uniekedagen. If so, you can use the date from that entity to constrain the list of appointments being returned. I am not sure what attributes you have in that entity, but I will assume you only have one date attribute. To constrain you could:
create a variable for the start of the day using the function trimToDays($Rooster_Uniekdagen/DateAttribute)
create a second variable for the start of the next day using the function addDays(trimToDays($Rooster_Uniekedagen/DateAttribute),1)
In the retrieve, use xpath to constrain the dates of the the appointments retrieved (something like ApptDateStart >= $StartOfDay and ApptDateStart < $StartOfNextDay ) You'll need to decide what conditions you want to use, i.e. should the appt start in a given day, start and end in a given day, what happens if an appointment spans 2 days, etc.