Validating data in excel carbon dating formula

The ISNUMBER function can be used to create a validation rule that only allows the entry of numeric values in a cell.Select Custom from the Allow list and use the formula below.

In this example, the items were in range A1: A5 of a sheet called Names.The formula below counts the occurrences of the inputted value in the range A2: A8.If the answer is 0 then the value is unique and allowed.In this example, we prevent the entry of a future date.Maybe users are recording a transaction that has occurred.

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