Arnold,
You are missing a End if and End Select in your code
Try this. It worked for me.
However if it is blank you get your error message then error 1004.
I put in a error handle for error 1004

On Error GoTo Handler
If mydate = "" Then
x = MsgBox("blank error message", vbCritical, "Error")

Range("PeriodFrom").Select
Range("PeriodFrom").Activate
End
End If

Exit Sub
Handler:
Select Case Err.Number
Case 1004
Err.Clear
Case 13 'data type mismatch
x = MsgBox("Date format error message", vbCritical, "Error")
Range("PeriodFrom").Select
Range("PeriodFrom").Activate
End
Case Else

End Select