os.time - How to use it?

Time is a very difficult to rigorously define.

The standard way that it is done is by counting the number of seconds from some particular, special moment in history.

tick() counts the number of seconds since the beginning of 1970 in your time zone.

os.clock() does a similar thing. However, it measures universal time meaning it references the beginning of 1970 GMT, regardless of which timezone the computer is in.

os.clock() is also an integer, while tick() provides a fraction of a second.

Here's a function that computes the day, year, etc, approximately, given the two functions:

-- The rule for whether or not `year` is a leap year.
function isleap(year)
	if year % 400 == 0 then
		return true;
	elseif year % 100 == 0 then
		return false;
	elseif year % 4 == 0 then
		return true;
	end
	return false;
end

function calendar( t )
	-- t is seconds since beginning of 1970
	local minutes = t / 60;
	local hours = minutes / 60;
	local days = hours / 24;
	local week = (days % 7 + 4) % 7;
	-- Weekday; 0 Su, 1 Mo, 6 Sa.
	-- Year / month is annoying because of leapyears:
	local dayStart = 0;
	local year;
	for y = 1970, 2100 do
		local len = 365;
		if isleap(y) then
			len = 366;
		end
		dayStart = dayStart + len;
		if dayStart > days then
			year = y;
			dayStart = dayStart - len;
			break;
		end
	end
	hours = hours % 24;
	minutes = minutes % 60;
	days = days - dayStart;
	local mos = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	local monthStart = 0;
	if isleap(year) then
		mos[2] = 29;
		days = days % 366;
	else
		days = days % 365;
	end
	for mo = 1, 12 do
		monthStart = monthStart + mos[mo];
		if monthStart > days then
			monthStart = monthStart - mos[mo];
			month = mo;
			break;
		end
	end
	local date = days - monthStart;
	print("Today's Date:");
	local monthName = {
		"January", "February", "March", "April", "May", "June", "July",
		"August", "September", "October", "November", "December"
	};
	print(math.floor(date) + 1, monthName[month], year);
	print(math.floor(hours), ":", math.floor(minutes));
end

calendar( os.time() );
--Today's Date:
-- 12 December 2014
-- 19 : 40

calendar( tick() );
-- Today's Date:
-- 12 December 2014
-- 14 : 40

os.time() results in the time in GMT (English time).

tick() results in my local time (EST)

Time is bad. If you can, avoid dealing with it. Using HTTP service to query some web service about time information might almost be a better option.

Or, finding a library that deals with it properly (I'm sure there is one, somewhere)