How to show compare date alert message in pop up window?

See more:

C#

ASP.NET

asp:CompareValidatorID="cmpVal1"ControlToCompare="txtStartDate"
    ControlToValidate="txtEndDate"Type="Date"Operator="GreaterThanEqual"
    ErrorMessage="*InvalidData"runat="server">

I had compare start date and end date in above code.Now i want to show alert message in pop window.how to show compare date alert in pop up window.

What I have tried:

I have compare two dates and get the compare message but the message is not shown in pop up window how to show in pop up window.