Replace and Substitute will both swap out cell content, but they do it differently. The Substitute function will find content you search for, and the Replace function will count characters. Sometimes you want one method and sometimes you want the other. This video shows how they both work:
Between the Sheets — Excel podcast episode 31: Excel’s secret DATEDIF function
A quick and simple way of finding the number of days, months or years between two dates is Excel’s secret DATEDIF function. When you type it in a worksheet, there’s no popup syntax help to indicate the function exists.