custom validator to check if IsDate works in IE but does not fire in firefox

I have a textbox with a custom validator to check if the users input is a date format or can be converted to a date. I am using the IsDate() function for clientside validation as well as serverside validation. (see code snipper below). It works fine in IE. when I type in some alpha numeric chracters the custom validator fires and show a message in the ValidationSummary  "this is not a valid date". In Firefox is does not do that. Please advice what am I doing wrong here. Thanks.
In the master page:
    <script type="text/vbscript">
    Function ValidateDate(source,args)
        if not IsDate(args.Value) then
            args.IsValid = False
            args.IsValid = True
        End if
     End function
The custom validator:
    <asp:CustomValidator ID="CustomValidator1" runat="server"
        ControlToValidate="CheckInCheckOut" ErrorMessage="Date is not valid" 
        ClientValidationFunction="ValidateDate" Display="None">
Code behind the web page:
    Protected Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles CustomValidator1.ServerValidate
        If Not IsDate(args.Value) Then
            args.IsValid = False
            args.IsValid = True
        End If
    End Sub

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PdeLormeAuthor Commented:
I have found in the meantime an other way to solve this with compare validator and its works fine. Thanks.

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