Find difference between 2 dates in JavaScript using format yyyy-mm-dd

jsdatediff2.js

function getNewDate(dateString){

  var date = dateString ? new Date(dateString) : new Date(),
      timezoneOffset = date.getTimezoneOffset();

      return new Date( date.getTime() + timezoneOffset*60*1000*(dateString?1:0) );
}

var start_date = getNewDate('2013-12-01'),
    end_date   = getNewDate(),
    one_day_in_milliseconds = 1000*60*60*24,
    date_diff = Math.floor( ( end_date.getTime() - start_date.getTime() )/one_day_in_milliseconds ) ;
    
    console.log("Date difference between ",end_date, " & ",start_date, " - ", date_diff);