function longDate(date) {
  var monthsArray = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'];
  var dateArray = date.split('/');
  var month = dateArray[0];
  var day = dateArray[1];
  var year = dateArray[2];

  if(year.length > 2) {
    if(Number(year) <= 20)
    year = '19' + dateArray[2];
  } else {
    year = '20' + dateArray[2];
  }

  return `${monthsArray[month - 1]} ${day}, ${year}`;
}

console.log(longDate('12/25/2016'));   // => "December 25, 2016"
console.log(longDate('4/1/16'));  // => "April 1, 2016"
console.log(longDate('7/12/55'));    // => "July 12, 1955"