DATE_ADD function

In appointment BO I have a rule:

If Appointment.StartTime IS DEFINED Then
Appointment.EndTime=BUSINESS_DAY_FORWARD(ThisAppointment.StartTime)

It works fine for me, but I dont want BUSINESS_DAY_FORWARD, I want just DAY_FORWARD.
So I used:

If Appointment.StartTime IS DEFINED Then
Appointment.EndTime=DATE_ADD(ThisAppointment.StartTime,1)

And expected it to set Appointment.EndTime to the next day after StartTime, but it sets it to the same date.

Is semantics in last rule wrong?

StartTime is a Timestamp. Adding 1 to a timestamp adds one hour, not one day. Try this:
Appointment.EndTime=DATE_PART(Appointment.StartTime)+1

The DATE_PART function converts a timestamp to a date.

It worked! Thanks a lot!
It was a timestamp but had a format dd-MM-yyyy thats why I confused.