Textbox Time Format Only in GridView

How do I modify the following code so the textbox will accept ONLY a valid time format e.g. 07:31       e.g.  12:50     e.g.  17:25

<asp:TemplateField ItemStyle-HorizontalAlign="Center"><ItemTemplate><asp:TextBox ID="txtTime" CssClass="TextCentered" MaxLength="5" runat="server"></asp:TextBox><ajaxToolkit:FilteredTextBoxExtender ID="ftbe" runat="server"
        TargetControlID="txtTime" FilterType="Custom, Numbers"
        ValidChars=":" /></ItemTemplate><HeaderTemplate>Execution Time</HeaderTemplate></asp:TemplateField>