Need to revise formula
 

In a chart where columns G and L are used to record the entry and exit dates,
column S reflects the number of weeks (difference). I used the formula
=(L-G)/7. This formula works when dates are entered in columns G and L.
However, if either cell is empty, I get the "value" error message. How can I
revise the formula that produces a 0 if columns G and L arte empty?

Thanks