Calculate next review date in excel

I have an excel spreadsheet with any and all due dates for the various things we need to track. One of which is next review due dates. I have columns for hire date, date of last review, date of next review, and # of days until due. I am trying to create a formula for the date of next review, but would like to keep the cell blank if there is no hire date in the proper column, and the next review date needs to change depending on the last date given. Can anyone help?