excel formula

I have a question. I am attempting to build a spreadsheet and I am kind of lost. Her is what my spread sheet looks like.

A1=asset number
B1=date pm performed
C1=frequency of pm (monthly, 6month, yearly)
D1=status of pm (comp=complete, in=in=progress)
E1=next pm due date

My question is, how do I writ a formula in E1 so when I enter “comp” in D1 the next pm date will show up in E1,(last pm + frequency = next pm).