strftime.js

/* Port of strftime() by T. H. Doan (https://thdoan.github.io/strftime/)
 *
 * Day of year (%j) code based on Joe Orost's answer:
 * http://stackoverflow.com/questions/8619879/javascript-calculate-the-day-of-the-year-1-366
 *
 * Week number (%V) code based on Taco van den Broek's prototype:
 * http://techblog.procurios.nl/k/news/view/33796/14863/calculate-iso-8601-week-and-year-in-javascript.html
 */
function strftime(sFormat, date) {
  if (typeof sFormat !== 'string') {
    return '';
  }

  if (!(date instanceof Date)) {
    date = new Date();
  }

  const nDay = date.getDay();
  const nDate = date.getDate();
  const nMonth = date.getMonth();
  const nYear = date.getFullYear();
  const nHour = date.getHours();
  const nTime = date.getTime();
  const aDays = [
    'Sunday',
    'Monday',
    'Tuesday',
    'Wednesday',
    'Thursday',
    'Friday',
    'Saturday'
  ];
  const aMonths = [
    'January',
    'February',
    'March',
    'April',
    'May',
    'June',
    'July',
    'August',
    'September',
    'October',
    'November',
    'December'
  ];
  const aDayCount = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334];
  const isLeapYear = () => (nYear % 4 === 0 && nYear % 100 !== 0) || nYear % 400 === 0;
  const getThursday = () => {
    const target = new Date(date);
    target.setDate(nDate - ((nDay + 6) % 7) + 3);
    return target;
  };
  const zeroPad = (nNum, nPad) => ((Math.pow(10, nPad) + nNum) + '').slice(1);

  return sFormat.replace(/%[a-z]+\b/gi, (sMatch) => {
    return (({
      '%a': aDays[nDay].slice(0, 3),
      '%A': aDays[nDay],
      '%b': aMonths[nMonth].slice(0, 3),
      '%B': aMonths[nMonth],
      '%c': date.toUTCString().replace(',', ''),
      '%C': Math.floor(nYear / 100),
      '%d': zeroPad(nDate, 2),
      '%e': nDate,
      '%F': (new Date(nTime - (date.getTimezoneOffset() * 60000))).toISOString().slice(0, 10),
      '%G': getThursday().getFullYear(),
      '%g': (getThursday().getFullYear() + '').slice(2),
      '%H': zeroPad(nHour, 2),
      '%I': zeroPad((nHour + 11) % 12 + 1, 2),
      '%j': zeroPad(aDayCount[nMonth] + nDate + ((nMonth > 1 && isLeapYear()) ? 1 : 0), 3),
      '%k': nHour,
      '%l': (nHour + 11) % 12 + 1,
      '%m': zeroPad(nMonth + 1, 2),
      '%n': nMonth + 1,
      '%M': zeroPad(date.getMinutes(), 2),
      '%p': (nHour < 12) ? 'AM' : 'PM',
      '%P': (nHour < 12) ? 'am' : 'pm',
      '%s': Math.round(nTime / 1000),
      '%S': zeroPad(date.getSeconds(), 2),
      '%u': nDay || 7,
      '%V': (() => {
        const target = getThursday();
        const n1stThu = target.valueOf();
        target.setMonth(0, 1);
        const nJan1 = target.getDay();

        if (nJan1 !== 4) {
          target.setMonth(0, 1 + ((4 - nJan1) + 7) % 7);
        }

        return zeroPad(1 + Math.ceil((n1stThu - target) / 604800000), 2);
      })(),
      '%w': nDay,
      '%x': date.toLocaleDateString(),
      '%X': date.toLocaleTimeString(),
      '%y': (nYear + '').slice(2),
      '%Y': nYear,
      '%z': date.toTimeString().replace(/.+GMT([+-]\d+).+/, '$1'),
      '%Z': date.toTimeString().replace(/.+\((.+?)\)$/, '$1'),
      '%Zs': new Intl.DateTimeFormat('default', {
        timeZoneName: 'short',
      }).formatToParts(date).find((oPart) => oPart.type === 'timeZoneName')?.value,
    }[sMatch] || '') + '') || sMatch;
  });
}