If you do not need to set the time part of the attribute as you said, you can simply assign the new date calculated by BUSINESS_DAY_FORWARD to EndTime attribute. The time part will be set to zero.
If you do need to preserve the time part when moving the date forward, you can do it like this:
Appointment.EndTime = Appointment.StartTime + DAY_DIFFERENCE(DATE_PART(Appointment.StartTime), BUSINESS_DAY_FORWARD(DATE_PART(Appointment.StartTime)))*24