Limiting Entry of Prior Dates

If you use the data validation capabilities of Excel, you can limit what goes into a cell, based on the contents of another cell. For instance, you can easily limit what goes into cell A2 based on a date that is in cell A1. Follow these steps:

1. Select cell A2.
2. Choose Validation from the Data menu. If you are using Excel 2007, click the Data tab of the ribbon then click Data Validation in the Data Tools group. Excel displays the Data Validation dialog box.
3. Make sure the Settings tab is displayed. (Click here to see a related figure.)
4. Using the Allow drop-down list, choose Date.
5. Using the Data drop-down list, choose Greater Than or Equal To.
6. In the Start Date box, enter =A1. This tells Excel that the date must be greater than or equal to whatever date is in cell A1.
7. Click OK.

Now, anytime you try to enter a date in cell A2 that is earlier than the date in cell A1, Excel displays an error message and will not allow the date to be entered.

What happens, however, when you want to limit the dates that can be entered in cell A1? For instance, if you put the date 4/1/04 in cell A1, and you want to make sure that the next date entered in A1 is not earlier than 4/1/04. If you put a date such as 4/15/04 in cell A1, that would be OK, but then the next time you enter a date in cell A1 you don't want it earlier than 4/15/04. In other words, you want to make sure that cell A1 can only accept dates later than the date currently in A1.

This is a bit stickier. If you follow the above steps but select cell A1 in step 1, then data validation won't work. Why? Because the date you enter in cell A1 will always be greater than or equal to the date you enter in A1--Excel doesn't compare to the previous date in A1 when doing data validation.

The only way to work through this problem is through the use of two macros. First, place the following macro in a regular module:

Sub Date_Validation()
    Dim dteDate As Date
    Dim strDate As String

    With Range("A1")
    '   Memo original date
        dteDate = CDate(.Text)
    '   Create date string
        strDate = Format(dteDate, "m\/d\/yy")
        With .Validation
        '   Delete old settings
            .Delete
        '   Set new data validation
            .Add _
              Type:=xlValidateDate, _
              AlertStyle:=xlValidAlertStop, _
              Operator:=xlGreaterEqual, _
              Formula1:=strDate

            .IgnoreBlank = False
            .InCellDropdown = True
            .InputTitle = ""
            .ErrorTitle = "Invalid Date Entry"
            .InputMessage = ""
            .ErrorMessage = _
              "Date is older than the previous date (" & _
              dteDate & ")."
            .ShowInput = True
            .ShowError = True
        End With
    End With
End Sub

This macro needs to be called by another macro, this one placed in the worksheet's code window, so that it is triggered every time there is a change in the worksheet:

Private Sub Worksheet_Change(ByVal Target As Range)
    On Error Resume Next
    If Target = Range("A1") Then Date_Validation
End Sub

The way these macros work is really quite interesting. Because you place the latter one in the worksheet's code window, it triggers every time there is a change to the worksheet. If the cell being changed is A1, then the Date_Validation macro is run.

The Date_Validation macro grabs the date from cell A1 and constructs a data validation rule for the cell. That's all it does--sets a data validation rule that won't allow a date to be entered in the cell that is earlier than the date currently in the cell.

The beauty of the macro is that once the data validation rule is in effect, then the next time cell A1 is changed, the data validation rule is triggered before the Worksheet_Change event is fired. Thus, the data validation rule makes sure that only a date greater than the current date can be entered. Once data validation is cleared, then the macro takes care of resetting the data validation rule, so it compares to the newly entered date.