Click on the calendar image near the textbox.
Change the date inside the textbox using format other than dd/MM/yyyy.
The field validator will raise an error.

<< Wednesday, September 27, 2023 >>

<ASP:TextBox runat="server" ID="txtDate"></ASP:TextBox>

<obout:Calendar runat="server"
                         DatePickerMode="true"
                         TextBoxId="txtDate">
</obout:Calendar>

<ASP:RequiredFieldValidator ID="RequiredFieldValidator1"
                         ControlToValidate="txtDate" runat="server" ErrorMessage="*">
</ASP:RequiredFieldValidator>

<ASP:RegularExpressionValidator ID="DateValidator" ControlToValidate="txtDate"
                         Display="Dynamic" EnableClientScript="true" runat="server"
                         ValidationExpression="^(0[1-9]|[12][0-9]|3[01])[/](0[1-9]|1[012])[/](19|20)\d\d$"
                         SetFocusOnError="true">
                         Please enter date in dd/MM/yyyy format
</ASP:RegularExpressionValidator>