HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<asp:TextBox runat="server" ID="txtDate" AutoPostBack="true" CausesValidation="true" />
<asp:CustomValidator ID="CustomValidator1" runat="server" ControlToValidate="txtDate"
ErrorMessage="Invalid Date format. Correct format dd/MM/yyyy" OnServerValidate="ValidateDateFormat" />
</form>
</body>
</html>
Code
protected void ValidateDateFormat(object sender, ServerValidateEventArgs e)
{
DateTime dateTime;
e.IsValid = DateTime.TryParseExact(e.Value, "dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None, out dateTime);
}