Column c may or may not have a date in each cell.
Column d may or may not have a date in each cell.
They will not have a date in corresponding cells at the same time.
If column c has a date then add 90 to that to equal column e
If column d has a date then add 60 to that to get column e.
And on it goes for each cell in each column

C1 D1 E1
3/27/2008 result
4/14/2008 result
C2 D2 E2
4/21/2008 result

Please help...of course column E would convert serial number to date.
MANY THANKS...
caseysmydog