IsDate can help you out there
You can do something like the following:
If ValidateDate(txtDate.Text) = False Then
MsgBox "Date is Invalid - please re-enter",vbOKOnly, "Invalid Date"
End If
Public Function ValidateDate(strDate as string) as Boolean
If IsDate(strdate) Then
ValidateDate = True
Else
ValidateDate = False
End If
End Function