Appointment BO, add minutes to the EndTime

Please advice, how would you :

Create an Appointment BO where user selects a StartTime
User selects from a dropdown an attribute ( Medical Treatment ) which may have different types of duration showned in numbers and representing minutes : 15, 30, 45
have the EndTime calculated like : Appointment.EndTime=Appointment.StartTime + Appointment.TreatmentDuration

I have tried to used MINUTES(Appointment.StartTime) but it does not work...

Managed to make it work using the TIME_ADD function, sample here :

https://forum.awareim.com/d/7962/3&hilit=MINUTES+add+minutes#p33728