Hi, trap the Form Error event, display your message and tell MS Access not
to display it's message:

Private Sub Form_Error(DataErr As Integer, Response As Integer)
   If DataErr = 2279 Then
       MsgBox "Text is not formatted correctly. Please re-enter"
       Cancel = acDataErrContinue
   End If
End Sub

You might also be able to parse the entered data and point out the specific
error.

HTH, Graeme.