I'm trying to create a sort of "validation if" in an excel sheet.

In cell A1, I have three choices, "A" "B" and "C" which a user can select from a drop down list. Let's say the user selects "A." I want to set up the validation so that if today is past a certain date (a date listed in cell B2), then cell A1 will automatically change to "B" instead of "A."




A B
1 a/b/c date
2
3
4


Any ideas?