format.js

Date.prototype.format = function(string) {
	// Helper function
	var pad = function(width, string, padding) { 
		return (width <= (string + "").length) ? (string + "") : pad(width, (padding + "") + (string + ""), (padding + ""))
	}

	// Build data lazily, only evaluate when needed.
	var date = this; // For scoping reasons.
	var leap = ((date.getFullYear() % 4 == 0) && (date.getFullYear() % 100 != 0)) || (date.getFullYear() % 400 == 0);
	var replacements = {
		/* Day */
		"d": function() {return pad(2, date.getDate(), 0)},
		"D": function() {return ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"][date.getDate()]},
		"j": function() {return date.getDate()},
		"l": function() {return ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"][date.getDate()]},
		"N": function() {return date.getDay() != 0 ? date.getDay() : 7},
		"S": function() {return ["th", "st", "nd", "rd", "th", "th", "th", "th", "th", "th"][(date.getDate() - 1) % 10]},
		"w": function() {return date.getDay()},
		"z": function() {return [0, 31, 31+ leap?29:28, 31+ leap?29:28 +31, 31+ leap?29:28 +31+30, 31+ leap?29:28 +31+30+31, 31+ leap?29:28 +31+30+31+30, 31+ leap?29:28 +31+30+31+30+31, 31+ leap?29:28 +31+30+31+30+31+31, 31+ leap?29:28 +31+30+31+30+31+31+30, 31+ leap?29:28 +31+30+31+30+31+31+30+31, 31+ leap?29:28 +31+30+31+30+31+31+30+31+30, 31+ leap?29:28 +31+30+31+30+31+31+30+31+30+31][date.getMonth()] + date.getDate() - 1},

		/* Week */
		"W": function() {return Math.ceil((((date - new Date(date.getFullYear(), 0, 4)) / 86400000) + new Date(date.getFullYear(), 0, 4).getDay() + 1) / 7)},
		
		/* Month */
		"F": function() {return ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"][date.getMonth()]},
		"m": function() {return pad(2, date.getMonth() + 1, 0)},
		"M": function() {return ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"][date.getMonth()]},
		"n": function() {return date.getMonth() + 1},
		"t": function() {return [31, leap?29:28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][date.getMonth()]},

		/* Year */
		"L": function() {return leap ? 0 : 1},
		"Y": function() {return date.getFullYear()},
		"y": function() {return date.getFullYear() % 100},

		/* Time */
		"a": function() {return date.getHours() > 11 ? "pm" : "am"},
		"A": function() {return date.getHours() > 11 ? "PM" : "AM"},
		"B": function() {return (date.getHours() * 360 + date.getMinutes() * 60 + date.getSeconds() + 1) / 8640},
		"g": function() {return date.getHours() % 12 + 1},
		"G": function() {return date.getHours()},
		"h": function() {return pad(2, date.getHours() % 12 + 1, 0)},
		"H": function() {return pad(2, date.getHours(), 0)},
		"i": function() {return pad(2, date.getMinutes(), 0)},
		"s": function() {return pad(2, date.getSeconds(), 0)},
		"v": function() {return date.getMilliseconds()},

		/* Timezone */
		"O": function() {return date.getTimezoneOffset() < 0 ? "-" : "+" + pad(2, Math.abs(date.getTimezoneOffset() / 60), 0) + pad(2, Math.abs(date.getTimezoneOffset() % 60), 0)},
		"P": function() {return date.getTimezoneOffset() < 0 ? "-" : "+" + pad(2, Math.abs(date.getTimezoneOffset() / 60), 0) + ":" + pad(2, Math.abs(date.getTimezoneOffset() % 60), 0)},
		"Z": function() {return date.getTimezoneOffset() * 60},

		/* Full */
		"c": function() {return date.toISOString()},
		"U": function() {return date.now / 1000},

		/* Other */
		"%": "%"
	}

	// Format the string
	string = string.split("");
	var output = "";

	for(var i = 0; i < string.length; i++) {
		if(string[i] == "%" && string[i + 1] in replacements) {
			// Lazy evaluation
			if(typeof replacements[string[++i]] === "function") replacements[string[i]] = replacements[string[i]]();
			output += replacements[string[i]];
		} else {
			output += string[i];
		}
	}

	return output;
}