JSFiddle - React, Tailwind, and code Playground

JavaScript

function humanise (diff) {
  // The string we're working with to create the representation
  var str = '';
  // Map lengths of `diff` to different time periods
  var values = [[' year', 365], [' month', 30], [' day', 1]];

  // Iterate over the values...
  for (var i=0;i<values.length;i++) {
    var amount = Math.floor(diff / values[i][1]);

    // ... and find the largest time value that fits into the diff
    if (amount >= 1) {
      // If we match, add to the string ('s' is for pluralization)
      str += amount + values[i][0] + (amount > 1 ? 's' : '') + ' ';

      // and subtract from the diff
      diff -= amount * values[i][1];
    }
  }

  return str;
}

alert(humanise(35));