Regarding Date/Time Formula Field

Regarding Date/Time Formula Field

I have created one Date/Time formula field called Appointment Date time with IF condition.

 ie

[ If(Dayofmonth(${Leads.Rescheduled Appointment Date/Time})==0,${Leads.Original Appointment Date/Time},${Leads.Rescheduled Appointment Date/Time}) ]

This formula field should show the value either 'rescheduled appointment date' (if rescheduled appointment date is filled) or Original Appointment date (if rescheduled appointment date is empty) ,but it is working only when two fields (original appnt and rescheduled appnt) are filled How can I resolve this problem.

Can anybody help me out by appropriate solution?