Timezone-aware ISO String in JavaScript

output.txt

$ node
Welcome to Node.js v14.16.1.
Type ".help" for more information.
> const { getTimezoneAwareISOString } = require('./time')
undefined
> getTimezoneAwareISOString()
'2021-06-13T15:42:58.304-05:00'
> new Date().toISOString()
'2021-06-13T20:43:04.271Z'
> getTimezoneAwareISOString(new Date(2020, 11, 15, 12, 15, 12, 100))
'2020-12-15T13:15:12.100-05:00'

time.js

/**
 * Retrieves the current time or converts a Date into
 * an ISO timezone aware string. i.e '2021-06-13T09:22:15.926-05:00'
 * By default, new Date().toISOString() would be UTC.
 *
 * @param {Date|null} date - Optionally, the date to
 *     convert to a timezone aware ISO string. Otherwise,
 *     the current time will be used.
 *
 * @returns {String} A timezone aware ISO date string.
 */
function getTimezoneAwareISOString(date = null) {
  let tzOffsetMin = new Date().getTimezoneOffset();
  let tzOffsetMs = tzOffsetMin * 60000;
  let timeMs = date instanceof Date ? date.getTime() : Date.now();

  return new Date(timeMs - tzOffsetMs).toISOString().replace(
    'Z',
    // A '-' is used here since the offset sign is
    // flipped from what we want when we
    // getTimezoneOffset(). Ex. UTC+10 => -600
    // However, we want +10:00 in the ISO string,
    // not -10:00.
    convertMinutesToOffset(-tzOffsetMin)
  );
}

/**
 * Convert minutes to a UTC offset.
 *
 * @param {Number} minutes - The minutes to convert to an offset.
 * @returns {String} A time of the form '+10:00'.
 */
function convertMinutesToOffset(minutes) {
  if (minutes === 0) {
    return 'Z';
  }

  let absMinutes = Math.abs(minutes);
  let h = Math.floor(absMinutes / 60);
  let m = absMinutes % 60;
  h = h < 10 ? '0' + h : h;
  m = m < 10 ? '0' + m : m;

  let hoursMins = h + ':' + m;
  return minutes > 0 ? `+${hoursMins}` : `-${hoursMins}`;
}

module.exports = { getTimezoneAwareISOString };