Rick's solution didn't work, but again, I think it was because the
help I tried to give with the columns and rows wasn't clear.

Alright, assuming I got the column wrong (I still think the formula is
basically correct), put the following formula in D6 and copy it down (you
can copy it past your existing data if you want)...

=IF(C6="","",IF(C6<=B$1,D5-$H$4,INDEX(B$10:B$1000,1+DATEDIF(B$1,C6,"m"))))

Rick