Coumn A has dates and blank cells.
Coumn B has dates.

Is it possible to specify that a date entered into column A must be later
than a date in Column B? A problem arises when a cell in column A is blank,
because excel reads this as 0 and therefore ANY date in the corresponding
cell in column B is going to be perceived by excel as later and therefore not
valid. So, in other words, what I want is a to specify that if there is a
date in column A AND a date in column B, excel will accept only a date in
column B later than the date in column A. But if column A is blank, then
Column B will accept any date.

Thanks for assistance.
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William