Convert/Format Date to Serial Number not Date number.

Hello,

I'm trying to execute this code but to no avail. I get Run-time error 13 with Type mismatch.

Private Sub TextBox2_Change()
Dim cbBox As Long
Dim stLng As Long

cbBox = Me.ComboBox9.Text
stLng = CLng(cbBox)

Me.TextBox8.Text = Me.ComboBox3 & "_" & stLng
End Sub

ComboBox9 stores date which has been formatted to "Mmm yyyy" as per

ComboBox9.Value = Format(ComboBox9.Value, "Mmm yyyy")

.So I would like to convert it to long date serial number to be able to make a comparison later on.

What am I doing incorrectly here?

Thank you.