Planned maintenance scheduler

Hi All,

So I'm making a maintenance scheduler and its start up and some dates completed dates I don't know yet but I have frequency so ive put a formula of completed dates plus date frequency = next due date sand then. ive conditional formatted so if It goes over date then goes red but as you can se below where I don't have a completed date inputted it comes up red but in this case I would like it to be blank and not show any due date. anyone know the formula to do such thing I'm not very savvy with excel and tried sum if blanks and what not with or's but still unable to grasp it.

Thanks,
Ahmet