JavaScript provides a bunch of methods for getting and setting parts of a date object, but it lacks a simple way to format dates and times according to a user-specified mask.

PHP's date function and ColdFusion's discrete dateFormat and timeFormat functions provide good approaches to follow.

PHP uses a crazy mix of letters to represent various date entities. It does offer the advantages that you can apply both date and time formatting with one function, and that none of the special characters overlap.

ColdFusion on the other hand have conflicting letters where m and mm mean different things depending on whether you're dealing with dates or times. On the other hand, ColdFusion uses very easy to remember special characters for masks.

With my date formatter, I've tried to take the best features from both, and add some sugar of my own. It did end up a lot like the ColdFusion implementation though, since I've primarily used CF's mask syntax.

var now = new Date();
 
now.format("m/dd/yy");
// Returns, e.g., 6/09/07
 
// Can also be used as a standalone function
dateFormat(now, "dddd, mmmm dS, yyyy, h:MM:ss TT");
// Saturday, June 9th, 2007, 5:46:21 PM
 
// You can use one of several named masks
now.format("isoDateTime");
// 2007-06-09T17:46:21
 
// ...Or add your own
dateFormat.masks.hammerTime = 'HH:MM! "Can\'t touch this!"';
now.format("hammerTime");
// 17:46! Can't touch this!
 
// When using the standalone dateFormat function,
// you can also provide the date as a string
dateFormat("Jun 9 2007", "fullDate");
// Saturday, June 9, 2007
 
// Note that if you don't include the mask argument,
// dateFormat.masks.default is used
now.format();
// Sat Jun 09 2007 17:46:21
 
// And if you don't include the date argument,
// the current date and time is used
dateFormat();
// Sat Jun 09 2007 17:46:22
 
// You can also skip the date argument (as long as your mask doesn't
// contain any numbers), in which case the current date/time is used
dateFormat("longTime");
// 5:46:22 PM EST
 
// And finally, you can convert local time to UTC time. Either pass in
// true as an additional argument (no argument skipping allowed in this case):
dateFormat(now, "longTime", true);
now.format("longTime", true);
// Both lines return, e.g., 10:46:21 PM UTC
 
// ...Or add the prefix "UTC:" to your mask.
now.format("UTC:h:MM:ss TT Z");
// 10:46:21 PM UTC

No equivalent in CF.|

It is easy to change these or add your own.

See http://blog.stevenlevithan.com/archives/date-time-format

/*
 * Date Format 1.2.3
 * (c) 2007-2009 Steven Levithan <stevenlevithan.com>
 * MIT license
 *
 * Includes enhancements by Scott Trenda <scott.trenda.net>
 * and Kris Kowal <cixar.com/~kris.kowal/>
 *
 * Accepts a date, a mask, or a date and a mask.
 * Returns a formatted version of the given date.
 * The date defaults to the current date/time.
 * The mask defaults to dateFormat.masks.default.
 */
 
var dateFormat = function () {
  var token = /d{1,4}|m{1,4}|yy(?:yy)?|([HhMsTt])\1?|[LloSZ]|"[^"]*"|'[^']*'/g,
      timezone = /\b(?:[PMCEA][SDP]T|(?:Pacific|Mountain|Central|Eastern|Atlantic) (?:Standard|Daylight|Prevailing) Time|(?:GMT|UTC)(?:[-+]\d{4})?)\b/g,
      timezoneClip = /[^-+\dA-Z]/g,
      pad = function (val, len) {
              val = String(val);
              len = len || 2;
              while (val.length < len) val = "0" + val;
              return val;
            };
 
  // Regexes and supporting functions are cached through closure
  return function (date, mask, utc) {
    var dF = dateFormat;
 
    // You can't provide utc if you skip other args (use the "UTC:" mask prefix)
    if (arguments.length == 1 && Object.prototype.toString.call(date) == "[object String]" && !/\d/.test(date)) {
      mask = date;
      date = undefined;
    }
 
    // Passing date through Date applies Date.parse, if necessary
    date = date ? new Date(date) : new Date;
    if (isNaN(date)) throw SyntaxError("invalid date");
 
    mask = String(dF.masks[mask] || mask || dF.masks["default"]);
 
    // Allow setting the utc argument via the mask
    if (mask.slice(0, 4) == "UTC:") {
      mask = mask.slice(4);
      utc = true;
    }
 
    var	_ = utc ? "getUTC" : "get",
        d = date[_ + "Date"](),
        D = date[_ + "Day"](),
        m = date[_ + "Month"](),
        y = date[_ + "FullYear"](),
        H = date[_ + "Hours"](),
        M = date[_ + "Minutes"](),
        s = date[_ + "Seconds"](),
        L = date[_ + "Milliseconds"](),
        o = utc ? 0 : date.getTimezoneOffset(),
        flags = {
           d: d,
          dd: pad(d),
         ddd: dF.i18n.dayNames[D],
        dddd: dF.i18n.dayNames[D + 7],
           m: m + 1,
          mm: pad(m + 1),
         mmm: dF.i18n.monthNames[m],
        mmmm: dF.i18n.monthNames[m + 12],
          yy: String(y).slice(2),
        yyyy: y,
           h: H % 12 || 12,
          hh: pad(H % 12 || 12),
           H: H,
          HH: pad(H),
           M: M,
          MM: pad(M),
           s: s,
          ss: pad(s),
           l: pad(L, 3),
           L: pad(L > 99 ? Math.round(L / 10) : L),
           t: H < 12 ? "a"  : "p",
          tt: H < 12 ? "am" : "pm",
           T: H < 12 ? "A"  : "P",
          TT: H < 12 ? "AM" : "PM",
           Z: utc ? "UTC" : (String(date).match(timezone) || [""]).pop().replace(timezoneClip, ""),
           o: (o > 0 ? "-" : "+") + pad(Math.floor(Math.abs(o) / 60) * 100 + Math.abs(o) % 60, 4),
           S: ["th", "st", "nd", "rd"][d % 10 > 3 ? 0 : (d % 100 - d % 10 != 10) * d % 10]
        };
 
    return mask.replace(token, function ($0) {
      return $0 in flags ? flags[$0] : $0.slice(1, $0.length - 1);
    });
  };
}();
 
 
// Some common format strings
dateFormat.masks = {
  "default":      "ddd mmm dd yyyy HH:MM:ss",
  shortDate:      "m/d/yy",
  mediumDate:     "mmm d, yyyy",
  longDate:       "mmmm d, yyyy",
  fullDate:       "dddd, mmmm d, yyyy",
  shortTime:      "h:MM TT",
  mediumTime:     "h:MM:ss TT",
  longTime:       "h:MM:ss TT Z",
  isoDate:        "yyyy-mm-dd",
  isoTime:        "HH:MM:ss",
  isoDateTime:    "yyyy-mm-dd'T'HH:MM:ss",
  isoUtcDateTime: "UTC:yyyy-mm-dd'T'HH:MM:ss'Z'"
};
 
 
// Internationalization strings
dateFormat.i18n = {
  dayNames: [
    "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat",
    "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"
  ],
  monthNames: [
    "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
    "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"
  ]
};
 
 
// For convenience...
Date.prototype.format = function (mask, utc) {
  return dateFormat(this, mask, utc);
};

Note that the day and month names can be changed (for internationalization or other purposes) by updating the dateFormat.i18n object.