strftime - JavaScript implementation for strftime() function of ISO C90 + α

strftime.js

const en_US = {
    days: ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"],
    shortDays: ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"],
    months: ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"],
    shortMonths: ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"],
    AM: "AM",
    PM: "PM",
};

const literal = {
    "%%": "%",
    "%n": "\n",
    "%t": "\t",
};

const gen = (locale) => {
    return (format, dt) => {
        return format.replace(/%(-)?([a-zA-Z%])/g, ($$, $1, $2) => {
            let val = literal[$$];
            if (val)
                return val;
            const pad = (len, num) => ($1) ? num : ("000000000" + num).substr(-len, len);
            switch ($2) {
                /**
                 * %a     The abbreviated weekday name according to the current locale. (en-only)
                 */
                case "a":
                    return locale.shortDays[dt.getDay()];
                /**
                 * %A     The full weekday name according to the current locale. (en-only)
                 */
                case "A":
                    return locale.days[dt.getDay()];
                /**
                 * %b     The abbreviated month name according to the current locale. (en-only)
                 */
                case "b":
                    return locale.shortMonths[dt.getMonth()];
                /**
                 * %B     The full month name according to the current locale. (en-only)
                 */
                case "B":
                    return locale.months[dt.getMonth()];
                /**
                 * %C     The century number (year/100) as a 2-digit integer. (SU)
                 */
                case "C":
                    return pad(2, Math.floor(dt.getFullYear() / 100));
                /**
                 * %c     The preferred date and time representation for the current locale. [NOT IMPLEMENTED]
                 * %d     The day of the month as a decimal number (range 01 to 31).
                 */
                case "d":
                    return pad(2, dt.getDate());
                /**
                 * %H     The hour as a decimal number using a 24-hour clock (range 00 to 23).
                 */
                case "H":
                    return pad(2, dt.getHours());
                /**
                 * %I     The hour as a decimal number using a 12-hour clock (range 01 to 12).
                 */
                case "I":
                    return pad(2, ((dt.getHours() + 11) % 12) + 1);
                /**
                 * %L - Millisecond of the second (000..999)
                 */
                case "L":
                    $1 = null;
                    return pad(3, dt.getMilliseconds());
                /**
                 * %m     The month as a decimal number (range 01 to 12).
                 */
                case "m":
                    return pad(2, dt.getMonth() + 1);
                /**
                 * %M     The minute as a decimal number (range 00 to 59).
                 */
                case "M":
                    return pad(2, dt.getMinutes());
                /**
                 * %p     Either "AM" or "PM"
                 */
                case "p":
                    return dt.getHours() < 12 ? locale.AM : locale.PM;
                /**
                 * %S     The second as a decimal number (range 00 to 60)
                 */
                case "S":
                    return pad(2, dt.getSeconds());
                /**
                 * %U     The week number [NOT IMPLEMENTED]
                 * %y     The year as a decimal number without a century (range 00 to 99).
                 */
                case "y":
                    return pad(2, dt.getFullYear() % 100);
                /**
                 * %Y     The year as a decimal number including the century.
                 */
                case "Y":
                    return pad(4, dt.getFullYear());
                /**
                 * %z     The +hhmm or -hhmm numeric timezone (that is, the hour and minute offset from UTC). (SU)
                 */
                case "z":
                    val = -dt.getTimezoneOffset();
                    let hour = Math.floor(val / 60);
                    let min = val % 60;
                    if (hour < 10)
                        hour = "0" + hour;
                    if (min < 10)
                        min = "0" + min;
                    return (val < 0 ? "-" : "+") + hour + min;
                default:
                    return $$;
            }
        });
    };
};

exports.strftime = gen(en_US);

Another implementation:
https://github.com/samsonjs/strftime
https://github.com/samsonjs/strftime/blob/master/lib/index.js