format date as a UTC date string - yyyy-mm-ddThh:mm:dd.mmsZ

utc-date-string.js

/**
 * @function formatUTCDateString accepts a string or a number and attempts to
 * create a YYYY-MM-DDTHH:MM:DD.MSZ string and return it. If the input cannot
 * be parsed into a valid date, the original value is returned.
 * 
 * Function also supports a quoted timestamp as the argument, tries to coerce
 * it to produce a valid date string.
 * 
 * Note the JavaScript's Date object does *not* accept a quoted timestamp, as
 * follows:
 * 
 *  new Date("1563845923981") returns "Invalid Date"
 *  Date.parse("1563845923981") returns NaN
 *
 * @param {*} item
 * @returns {string}
 */

export function formatUTCDateString(item) {
  // First try to parse item as a timestamp (number), then as datestring.
  var time = (/^\d+$/.test(item) && new Date(Number(item))) || new Date(item);

  if (!Date.parse(time)) {
    // If we can't parse date to a valid timestamp, punt.
    return item;
  }

  var utc = {
    year: time.getUTCFullYear(),
    month: time.getUTCMonth() + 1,
    date: time.getUTCDate(),
    hours: time.getUTCHours(),
    minutes: time.getUTCMinutes(),
    seconds: time.getUTCSeconds(),
    milliseconds: time.getUTCMilliseconds()
  }

  Object.keys(utc).forEach(key => {
    var value = utc[key];

    utc[key] = value < 10 ? '0' + value : value;
  });

  var utcDate = [
    utc.year,
    utc.month,
    utc.date
  ];

  var utcTime = [
    utc.hours,
    utc.minutes,
    utc.seconds
  ];

  return [
    utcDate.join('-'),
    'T',
    utcTime.join(':'),
    '.',
    time.getUTCMilliseconds(),
    'Z'
  ].join('');
}

utc-date-test.js

  /* format value in as yyyy-mm-ddThh:mm:ss.msZ UTC Date */

  describe('formatUTCDateString(value)', () => {

    const RE_UTC_STRING = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.\d{3}Z$/;

    it('should accept and format a Date timestamp', () => {
      var utc = formatUTCDateString(Date.now());

      expect(RE_UTC_STRING.test(utc)).toBe(true);
    });

    it('should accept and format a Date object', () => {
      var utc = formatUTCDateString(new Date());

      expect(RE_UTC_STRING.test(utc)).toBe(true);
    });

    it('should accept and format 2019-07-23T01:00:01.477Z', () => {
      expect(formatUTCDateString('2019-07-23T01:00:01.477Z')).toBe('2019-07-23T01:00:01.477Z');
    });

    it('should format 1563845923981', () => {
      expect(formatUTCDateString(1563845923981)).toBe('2019-07-23T01:38:43.981Z');
    });

    it('should format a *quoted* timestamp, "1563845923981"', () => {

      // new Date("1563845923981") returns "Invalid Date"
      // Date.parse("1563845923981") returns NaN

      expect(formatUTCDateString("1563845923981")).toBe('2019-07-23T01:38:43.981Z');
    });
  });