If you haven't learned firsthand, data validation is a pretty tricky thing to do in Javascript. There is no built in isDate() function, so you pretty much has to rely on your Google searching skills to find some validation code. Or, look in some Javascript book. Or, in the worse case scenario, actually write some yourself. But thankfully, there's a much more elegant solution to this and most other problems: WebServices.

This way, as the anonymous submitter discovered in a system he was maintaining, you can simply make a call over Ethernet to make a call over IP to make a call over TCP to make a call over HTTP to make a call over XML to make a call to IIS to call the .NET Runtime which calls the VisualBasic runtime to run it's built-in IsDate function. 

function isDate(str)
{
  var isValid = "<result>false</result>";
  
  var webSvc = new WebService("dataValidation.asmx");
  webSvc.method = "IsDate";
  webSvc.parameters.push(str);

  if (webSvc.call())
  {
    isValid = webSvc.responseXml;
  }

  return isValid;
}
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