Port of php.js date to lua http://phpjs.org/functions/date/

date.lua

function date (format, timestamp) 
  -- @credits https://raw.github.com/kvz/phpjs/master/functions/datetime/date.js

	local c = {}
	local f = nil
	local formatPattern = "%a"

	local pad = function (v, s) 
		v = tostring(v)
		if string.len(v) < s then
			return c.pad("0" .. v, s)
		end
		
		return v
	end
	c.pad = pad
	
	local unpad = function(v)
		v = string.gsub(v, "^0", "")
		if string.find(v, "^0") then
			return c.unpad(v)
		end
		
		return v
	end
	c.unpad = unpad
	
	local parseInt = function (v)
		return math.floor(tonumber(v, 10))
	end
	
   	local round = function (num) 
		if num >= 0 then 
			return math.floor(num + .5) 
		else 
			return math.ceil(num - .5) 
		end
    	end

	local txt_ordinals = {
		'st', 'nd', 'rd', 'th'
	}

	local formatChrCb = function (chr)
		if f[chr] ~= nil then
			return f[chr]()
		end
		
		return chr
	end
	
	f = {
		-- [Day]--
		
		-- Day of month w/leading 0; 01..31
		d = function () 
			return os.date("%d", c.time)
		end,
		
		-- shorthand day name; Mon..Sun
		D = function () 
			return os.date("%a", c.time)
		end,
		
		-- Day of month; 1..31
		j = function () 
			return unpad(f.d())
		end,
		
		-- Full day name; Monday...Sunday
		l = function ()
			return os.date("%A", c.time)
		end,
		
		-- ISO-8601 day of week; 1[Mon]..7[Sun]
		N = function ()
			local d = f.w()
			return (parseInt(d) > 0 and d or "7")
		end,
		
		-- Ordinal suffix for day of month; st, nd, rd, th
		S = function () 
			local j = parseInt(f.j())
			local i = j % 10
			
			if (i <= 3) and (parseInt((j % 100) / 10) == 1) then
				i = 1
			end
			
			if i > 4 then
				i = 4
			end
			
			return txt_ordinals[i]
		end,
		
		--  Day of week; 0[Sun]..6[Sat]
		w = function ()
			return os.date("%w", c.time)
		end,
		
		-- Day of year; 0..365
		z = function ()
			local t = os.date("*t", c.time)
			return tostring(t.yday - 1)
		end,
		
		-- [Week] --
		W = function ()
			local m = parseInt(f.n()) -- - 1
			local d = parseInt(f.j()) - parseInt(f.N()) + 3
			
			local t  = os.time({year = f.Y(), month = m, day = d})
			local t2 = os.time({year = os.date("%Y", t), month = 1, day = 4})
			
			local v = 1 + round((t - t2) / 864e2 / 7);
			return pad(v, 2)		
		end,
		-- [Month] --
		
		-- Full month name; January...December
		F = function ()
			return os.date("%B", c.time)
		end,
		
		-- Month w/leading 0; 01...12
		m = function ()
			return os.date("%m", c.time)
		end,
		
		-- Shorthand month name; Jan...Dec
		M = function ()
			return os.date("%b", c.time)
		end,
		
		-- Month; 1...12
		n = function ()
			return unpad(f.m())
		end,
		
		-- Days in month; 28..31
		t = function ()
			local y = parseInt(f.Y())
			local m = parseInt(f.n())
			
			if m == 12 then
				t = os.time({year = y + 1, month = 1, day = 0})
			else 
				t = os.time({year = f.Y(), month = f.n() + 1, day = 0})
			end
			
			return unpad(os.date("%d", t))
		end,
		
		-- [Year] --
		
		-- Is leap year?; 0 or 1
		L = function ()
			local j = parseInt(f.Y())
			if ((j % 4) == 0) and ((j % 100) ~= 0) or ((j % 400) == 0) then
				return "1"
			end
			
			return "0"
		end,
		
		-- ISO-8601 year
		o = function ()
			local n = parseInt(f.n())
			local W = parseInt(f.W())
			local Y = parseInt(f.Y())
			local o = 0
			
			if (n == 12) and (W < 9) then
				o = 1
			elseif (n == 1) and (W > 9) then
				o = -1
			end
			
			return tostring(Y + o)		
		end,
		
		-- Full year; e.g. 1980...2010
		Y = function ()
			return os.date("%Y", c.time)
		end,
		
		-- 
		y = function ()
			return os.date("%y", c.time)
		end,
		
		-- am or pm
		a = function ()
			return string.lower(f.A())
		end,
		
		-- AM or PM
		A = function ()
			return os.date("%p", c.time)
		end,
		
		-- Swatch Internet time; 000..999
		-- TODO: check this function more precisely
		B = function () 
			local H = parseInt(f.H()) * 36e2 -- hours
			local i = parseInt(f.i()) * 60   -- minutes
			local s = parseInt(f.s())        -- seconds
			
			local v = ((H + i + s + 36e2) / 86.4) % 1e3
			return tostring(v)
		end,

		--  12-Hours; 1..12
		g = function ()
			return unpad(f.h())
		end,
		
		-- 24-Hours; 0..23
		G = function ()
			return unpad(f.H())
		end,
		
		-- 12-Hours w/leading 0; 01..12
		h = function ()
			return os.date("%I", c.time)
		end,
		
		--  24-Hours w/leading 0; 00..23
		H = function ()
			return os.date("%H", c.time)
		end,
		
		-- Minutes w/leading 0; 00..59
		i = function ()
			return os.date("%M", c.time)
		end,
		
		-- Seconds w/leading 0; 00..59
		s = function ()
			return os.date("%S", c.time)
		end,
			
		-- Microseconds; 000000-999000; will always return 000000
		u = function ()
			return "000000"
		end,
		
		-- Timezone
		e = function ()
			error("Not supported")
		end,
		
		-- DST observed?; 0 or 1
		I = function () 
			error("Not supported")
			-- -- Compares Jan 1 minus Jan 1 UTC to Jul 1 minus Jul 1 UTC.
			-- -- If they are not equal, then DST is observed.
			-- local a = os.time({year = f.Y(), month = 1, day = 1}), -- Jan 1
			-- local c = os.time({year = f.Y(), month = 1, day = 1}), -- Jan 1 UTC
			-- local b = os.time({year = f.Y(), month = 6, day = 1}), -- Jul 1
			-- local d = os.time({year = f.Y(), month = 6, day = 1}), -- Jul 1 UTC
			-- 
			-- if ((a - c) != (b - d) then
			-- 	return "1"
			-- end
			-- 
			-- return "0"
		end,
		
		-- Difference to GMT in hour format; e.g. +0200
		O = function ()
			error("Not supported")
		   -- local tzo = getTimezoneOffset()
		   -- local a = math.abs(tzo);
		   -- return (tzo > 0 ? "-" : "+") + pad(math.floor(a / 60) * 100 + a % 60, 4);
		end,
		
		-- Difference to GMT w/colon; e.g. +02:00
		P = function ()  
			error("Not supported")
			-- local O = f.O();
			-- return (O.substr(0, 3) + ":" + O.substr(3, 2));
			
		end,
		
		-- Timezone abbreviation; e.g. EST, MDT, ...
		T = function () 
			error("Not supported")
			-- return 'UTC'
		end,
					
		-- Timezone offset in seconds (-43200...50400)
		Z = function () 
			error("Not supported")
			--  return -getTimezoneOffset() * 60;
		end,
			
		--[ Full Date/Time]--
		-- ISO-8601 date.
		c = function ()
			return date('Y-m-d\\TH:i:sP', c.time)
		end,
		
		-- RFC 2822
		r = function ()  
			return date('D, d M Y H:i:s O', c.time)
		end,
		
		-- Seconds since UNIX epoch
		U = function ()
			return c.time
		end
	}

	c.date = function (format, timestamp) 
		c.time = timestamp or os.time()
		return string.gsub(format, formatPattern, formatChrCb)
	end

  return c.date(format, timestamp)
end