The "X" cell formulas below return a value based on
entries in cells U19 through U30 (12 months of a year).
The values for V10 and P10 may also change each month.
This system works fine for the immediate month being
posted; however, it returns a false history for previous
months, due to the changes in V10 and P10. I have to
maintain a history of the values returned in the "X"
cells - that is, once a value is entered in, say, U20, the
value in X19 (the previous month) is preserved -
regardless of new values entered in V10 and P10. Also, if
the user deletes an entry for a current month, the formula
works as before - as though no entry was ever made - kind
of a built-in undo.

There are 48 yearly ranges involved in this reporting
system, so I'm not sure if this is a cell formula or
module coding issue.

Is this possible? If so could someone help me through the
code?

Cell Formula
X19 =IF(U19="","",IF(U19<0,(U19-V10)/(P10-V10),0))
X20 =IF(U20="","",IF(U20<0,(U20-V10)/(P10-V10),0))
X21 =IF(U21="","",IF(U21<0,(U21-V10)/(P10-V10),0))
Etc.
X30 =IF(U30="","",IF(U30<0,(U30-V10)/(P10-V10),0))