Masked Edit Extender question

I am trying to use a masked edit extender for a text box that is to display time zone offset, in the form of 05:00 or -05:00, for example. The following is what I used and it initially displays the mask and allows me to enter either of the above values. But when I leave the textbox, it removes the colon and I get invalid format error. What am I missing here? (I changed the MaskType from Number to None hoping it would not drop : but it does).

<asp:TextBox runat="server" ID="tbTZOffset" CssClass="NormalSmall" Text="" /><ajaxToolkit:MaskedEditExtender runat="server" ID="meeTZO"
        TargetControlID="tbTZOffset"
        Mask="99:99"
        MessageValidatorTip="false"
        MaskType="None"
        InputDirection="LeftToRight"
        AcceptNegative="Left"
        ErrorTooltipEnabled="True"/><ajaxToolkit:MaskedEditValidator runat="server" ID="mevTZO"
        ControlExtender="mevTZO"
        ControlToValidate="tbTZOffset"
        IsValidEmpty="False"
        EmptyValueMessage="Time Zone Offset is required"
        InvalidValueMessage="Time Zone Offset is invalid (must be in [-]hh:mm format)"
        EmptyValueBlurredText="*"
        Display="none"
        ValidationExpression="\b[-]?\d{2}:\d{2}\b"
        TooltipMessage="Time Zone Offset in [-]hh:mm format"
        ValidationGroup="vgConfig"/>