Uses the new Intl API to format a date as a string.

formatIntlDate.js

var formatIntlDate = (() => {
  const CODE_TO_PROP_NAME = {
    Y: ['year', 'numeric'],
    M: ['month', 'long'],
    D: ['day', 'numeric'],
    WD: ['weekday', 'long'],
    h: ['hour', '2-digit'],
    m: ['minute', '2-digit'],
    s: ['second', '2-digit'],
    ms: ['fractionalSecondDigits', 3],
    DS: ['dateStyle', 'full'],
    TS: ['timeStyle', 'full'],
  };
  const CODES = Object.keys(CODE_TO_PROP_NAME).sort(
    (a, b) => a.length === b.length
      ? a < b ? -1 : 1
      : (b.length - a.length)
  ).join('|');
  const RGX = new RegExp(
    `%(${CODES})(?:<(numeric|2-digit|long|narrow|short|medium|1|2|3)>)?|([%<>])\\3`,
    'g'
  );

  /**
   * Uses the new Intl API to format a date as a string.
   * @param {Date} date
   * @param {?FormatIntlDateOptions=} opt_options
   * @returns {string}
   */
  function formatIntlDate(date, opt_options) {
    const {timeZone, format, locale, dateStyle, timeStyle} = opt_options ?? {};
    if ('string' === typeof format) {
      return format.replace(RGX, (m, id, format, escaped) => {
        if (escaped) return escaped;
        try {
          const [name, defaultValue] = CODE_TO_PROP_NAME[id];
          return new Intl.DateTimeFormat(locale, {
            [name]: format || defaultValue,
            timeZone
          }).format(date);
        } catch(e){}
        return m;
      });
    }
    return new Intl.DateTimeFormat(
      locale,
      {
        dateStyle: dateStyle ?? 'full',
        timeStyle: timeStyle ?? 'full',
        timeZone
      }
    ).format(date);
  }
  /**
   * @typedef FormatIntlDateOptions
   * @property {?string=} timeZone
   * @property {?string=} format
   * @property {?string=} locale
   * @property {?("full"|"long"|"medium"|"short")=} dateStyle
   * @property {?("full"|"long"|"medium"|"short")=} timeStyle
   */

  return formatIntlDate;
})();

~examples.js

console.log(
  formatIntlDate(new Date, {
    format: 'It is currently %WD<short> (AKA "%WD") in Adelaide.',
    timeZone: 'Australia/Adelaide'
  })
);

console.log(
  formatIntlDate(new Date, {
    format: 'It is currently %WD<short> (AKA "%WD") in Hawaii.',
    timeZone: 'US/Hawaii'
  })
);
  
for (const locale of ['en', 'en-AU', 'en-GB', 'en-US', 'es', 'es-419', 'es-BR', 'es-ES', 'es-MX', 'es-BO', 'es-AR', 'es-CO', 'pt', 'pt-BR', 'pt-PT', 'pt-AO', 'pt-CH', 'pt-CV', 'pt-GQ', 'pt-GW', 'pt-LU', 'pt-MO', 'pt-MZ', 'pt-ST', 'pt-TL', 'qu', 'qu-BO', 'qu-EC', 'qu-PE', 'sr', 'vi', 'vi-VN', 'sv', 'sw', 'zh', 'de', 'da', 'cs', 'ca', 'bo', 'bg', 'ar', 'af', 'el', 'fil', 'ga', 'fr', 'gl', 'haw', 'he', 'it', 'ko', 'mk', 'nds', 'nl', 'ru', 'sn', 'uk']) {
    console.log(`${locale}:\n\t${formatIntlDate(new Date, {locale})}`);
}