Is it acceptable to postpone my admission in a PhD program for a year? In my country, we have to do a year of military service after graduating. The service starts in January. If I applied for a PhD program and get accepted, I won't know till March or April, after I have entered the service. Will the university accept postponing my admission for another year? 

I will apply before the service, in case I get excused.
Postponing admission is likely to be approved, but it may depend on the finances of the program.  Usually this sort of decision would be made on a case-by-case basis.