JSFiddle - React, Tailwind, and code Playground

JavaScript

function pad(str) {
  if (str.length === 2) return str;
  return pad('0' + str);
}

function padDate(str, delimiter) {
  return str.split(delimiter).map(function (el) {
    if (el.length < 2) return pad(el);
    return el;
  }).join(delimiter);
}

console.log(padDate('9/17/2014', '/'));
console.log(padDate('19-7-2014', '-'));