Show alert message on wrong input

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C#

MVC

[HttpPost]
public ActionResult Index(string date)
{
    DateTime today = System.DateTime.Now;
    DateTime chk_date = Convert.ToDateTime(date);

    if (today < chk_date)

date is value which is send by textbox
if condition become true then it return a alert message box..

How i can do..

plz help me..

Thanx With Regards: