Render time interval in easy-to-read string.

interval_duration.js

/**
 * Compares two datetime values and returns a human-readable interval in past or
 * future tense.
 *
 * @param {string|number|Date} start The first (from) datetime to use in comparison.
 * @param {string|number|Date} stop The second (to) datetime to use in comparison.
 *
 * @example "6 seconds ago"
 *
 * @return {string} The interval statement.
 */
function interval_duration(start, stop)
{
  /**
   * @see convert_to_date_object https://gist.github.com/kmcconnell/f4931ee58edff1f593ee1f1a7f0ea51a
   */
  start = convert_to_date_object(start);
  stop  = convert_to_date_object(stop);

  var time_formats = [
    [60, 'seconds', 1],
    [120, '1 minute ago', '1 minute from now'],
    [3600, 'minutes', 60],
    [7200, '1 hour ago', '1 hour from now'],
    [86400, 'hours', 3600],
    [172800, 'yesterday', 'tomorrow'],
    [604800, 'days', 86400],
    [1209600, 'last week', 'next week'],
    [2419200, 'weeks', 604800],
    [4838400, 'last month', 'next month'], // 60*60*24*7*4*2
    [29030400, 'months', 2419200],
    [58060800, 'last year', 'next year'],
    [2903040000, 'years', 29030400],
    [5806080000, 'last century', 'next century'],
    [58060800000, 'centuries', 2903040000]
  ];

  var seconds = (stop - start) / 1000, token = 'ago', key = 1;

  if (seconds === 0) {
    return 'just now';
  }

  if (seconds < 0) {
    seconds = Math.abs(seconds);
    token = 'from now';
    key = 2;
  }

  var itx = 0, format;
  while (format = time_formats[itx++]) {
    if (seconds < format[0] && typeof(format[2]) == 'string') {
      return format[key];
    }
    if (seconds < format[0]) {
      return Math.floor(seconds / format[2]) + ' ' + format[1] + ' ' + token;
    }
  }
}