JavaScript Date Format Using PHP Wildcards

dateformat.js

Date.format = function(d, format, custom_months, custom_days){
	var months = custom_months || ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
		,days = custom_days || ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
		,replacements = [
				//day
				[ '%d', (d.getDate() < 10 ? '0' : '') + d.getDate() ] //leading 0: 01-31
			,	[ '%D', days[ d.getDay() ].substr(0, 3) ] //3 letter representation (Mon-Sun)
			,	[ '%j', d.getDate() ] //no leading 0: 1-31
			,	[ '%l', days[ d.getDay() ] ] //[lowercase L], full textual representation (Monday-Sunday)
			,	[ '%N', (d.getDay() + 1) ] //day #: 1-7 (1=monday, 7=sunday)

				//month
			,	[ '%F', months[ d.getMonth() ] ] //full textual representation (January-December)
			,	[ '%m', (d.getMonth() < 10 ? '0' : '') + (d.getMonth() + 1) ] //leading 0: 01-12
			,	[ '%M', months[ d.getMonth() ].substr(0, 3) ] //3 letter represenation (Jan-Dec)
			,	[ '%n', (d.getMonth() + 1) ] //no leading 0: 1-12

				//year
			,	[ '%Y', d.getFullYear() ] //4 digits 
			,	[ '%y', d.getFullYear().toString().substr(2) ] //2 digits

				//hour
			,	[ '%g', (d.getHours() % 12 == 0 ? 12 : d.getHours() % 12) ] //12 hour, without leading 0: 1-12
			,	[ '%G', (d.getHours() == 0 ? '00' : d.getHours()) ] //24 hour, without leading 0: 1-24
			,	[ '%h', (d.getHours() % 12 == 0 ? 12 : (d.getHours() < 10 ? '0' : '') ) + (d.getHours() % 12) ] //12 hour, leading 0: 01-12
			,	[ '%H', (d.getHours() == 0 ? 12 : (d.getHours() < 10 ? '0' : '')) + (d.getHours()) ] //24 hour, leading 0: 01-24

				//minute
			,	[ '%i', (d.getMinutes() < 10 ? '0' : '') + d.getMinutes() ] //with leading 0: 00-59

				//second
			,	[ '%s', (d.getSeconds() < 10 ? '0' : '') + d.getSeconds() ] //with leading 0: 00-59

				//ampm
			,	[ '%a', (d.getHours() < 12 ? 'am' : 'pm') ] //lowercase
			,	[ '%A', (d.getHours() < 12 ? 'AM' : 'PM') ] //uppercase
		]
		,i = null
	;

	for (i in replacements){
		format = format.replace( replacements[i][0], replacements[i][1] );
	}

	return format;
}

Attaching things to standard host Objects is risky; 3rd party code may overwrite your static property on Date.

Well, I kind of am that 3rd party selfishly overriding "Date.format"...but it's a fair point to be aware of if in a less controlled environment simply calling this date_format instead is probably a good idea.