Stolen from: http://jsfiddle.net/niklasvh/xfrLm/ a neat way to verify that a given date (in string format) is valid

isDate.js

/*
*  Takes input as date-dtring in the format of mm/dd/yyyy and verifies that it's a valid date
*  by creating a date object out of it and comparing the year/month/day to the given ones
*/
function isDate(date) {
   var objDate,  // date object initialized from the date string
       mSeconds, // date in milliseconds
       day,      // day
       month,    // month
       year;     // year
   // date length should be 10 characters (no more no less)
   if (date.length !== 10) {
       return false;
   }
   // third and sixth character should be '/'
   if (date.substring(2, 3) !== '/' || date.substring(5, 6) !== '/') {
       return false;
   }
   // extract month, day and year from the date (expected format is mm/dd/yyyy)
   // subtraction will cast variables to integer implicitly (needed
   // for !== comparing)
   month = date.substring(0, 2) - 1; // because months in JS start from 0
   day = date.substring(3, 5) - 0;
   year = date.substring(6, 10) - 0;
   // test year range
   if (year < 1000 || year > 3000) {
       return false;
   }
   // convert date to milliseconds
   mSeconds = (new Date(year, month, day)).getTime();
   // initialize Date() object from calculated milliseconds
   objDate = new Date();
   objDate.setTime(mSeconds);
   // compare input date and parts from Date() object
   // if difference exists then date isn't valid
   if (objDate.getFullYear() !== year ||
       objDate.getMonth() !== month ||
       objDate.getDate() !== day) {
       return false;
   }
   // otherwise return true
   return true;
}

console.log(isDate('1/1/2017')) // false - not in format mm/dd/yyyy
console.log(isDate('01/01/2017')) // true
console.log(isDate('13/01/2017')) // false - invalid date