If Not IsDate Seems Not To Work

I have the following:


Private Sub TextBox1_AfterUpdate()
'//Check for valid date
If Not IsDate(TextBox1) Then
Me.TextBox1 = ""
'Show SplashForm2 if not date
SplashForm2.Show
Me.TextBox1.SetFocus
End If
End Sub
I thought this was working but I put 22/07 in TextBox1 and stepping through it it goes from the "If Not statement" to the "End If"

Don't know why it is not recognizing that it is not a date. It is missing the back slash.