//Since JavaScripts counts months starting from 0 (January is 0),
//subtract 1 from the month integer submitted by the user

var formattedMonth = userSubmittedMonth - 1;
var testDate = new Date( userSubmittedYear,formattedMonth,userSubmittedDay,0,0,0,0) ;

if (
  testDate.getFullYear() != userSubmittedYear
  || testDate.getMonth() != formattedMonth
  || testDate.getDate() != userSubmittedDay
)
  {
    alert( "The date that you entered doesn't exist (like Feb. 31)" );
  }