format datetime to use year with two digits only

what happens when the date is a string entered by the user in a form text box?
how can the format be applied?

my textbox is: Me.txtDateMfg.Text

Applied when? When the data is posted to the server, or as the user is typing?

when the data is posted to the server?

one of the events on the for, is from a preview button, after date is entered and button gets clicked, this function happens:

 Protected Sub cvDateMfg_ServerValidate(source As Object, args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles cvDateMfg.ServerValidate
        If Date_IsValid(Me.txtDateMfg.Text) Then
            args.IsValid = True
        Else
            args.IsValid = False
        End If
    End Sub

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Is it possible to format the date right here, or before?

i got that one wrong, the first code that gets executed is:

   Protected Sub cvDateMfg_ServerValidate(source As Object, args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles cvDateMfg.ServerValidate
        If Date_IsValid(Me.txtDateMfg.Text) Then
            args.IsValid = True
        Else
            args.IsValid = False
        End If
    End Sub

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Then this code:

 Protected Sub cvDateMfg_ServerValidate(source As Object, args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles cvDateMfg.ServerValidate
        If Date_IsValid(Me.txtDateMfg.Text) Then
            args.IsValid = True
        Else
            args.IsValid = False
        End If
    End Sub

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where would it be the right spot to specify the format? Thank you for your help.

I am sorry for the many posts, I am getting frustrated a bit.

this is how I ended up doing it:

    Protected Sub cvDateMfg_ServerValidate(source As Object, args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles cvDateMfg.ServerValidate

        Dim dtDateMfg As DateTime = Nothing

        If DateTime.TryParse(Me.txtDateMfg.Text, dtDateMfg) Then
            Me.txtDateMfg.Text = dtDateMfg.ToString("MM/dd/yy")
        End If

        If Date_IsValid(Me.txtDateMfg.Text) Then
            args.IsValid = True
        Else
            args.IsValid = False
        End If
    End Sub

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Can you please let me know if you see anything wrong, or code that is not needed?

Thank you.