Hi..new to worksheet formula's and could use some guidance

i have created a worksheet with a formula in an entire row to calculate days
old.The formula looks at two cells and calculates the difference...thats the
easy part..works good =(c1-f14) etc

What i would like to know is
In the remaining cells down the row (containing formula) i get 38944 as a
value.even though there is no value in one of the date rows.Is there a way to
calculate only the populated rows and leave the others blank or with "0"

thanks in advance