So the other day, I found that Excel has a pretty useful function that isn't listed when you click on the function button. (see pics & try it yourself). It calculates the number of days, months, or years between two dates. (You could do weeks by using days and dividing by 7).
You can also find the number of days between by subtracting two dates.
DATEDIF(Start_Date, End_Date, Interval)