Re: os.date question

[Luiz Henrique de Figueiredo <lhf@...>]

> os.time() returns a time_t cast to a number. And time_t is very
> likely a 32 bit type on your machine. It counts in seconds, so it
> has a limited range, usually from 1970-2038 (signed 32 bit int).
> 
> I.e. you've found a (well known) limitation in your C library.

Note that time_t is signed. And indeed, os.date does work with negative times,
up till around 1901, which is consistent with signed 32-bit values
(2038-1970==1970-1902).

On my Linux box the program

	for i=1,200 do
		local t=365.25*24*60*60
		print(os.date(nil,-t*i))
	end

gives

Tue Dec 31 15:00:00 1968
Mon Jan  1 10:00:00 1968
Sun Jan  1 04:00:00 1967
...
Sun Dec 31 20:53:32 1905
Sat Dec 31 14:53:32 1904
Fri Jan  1 08:53:32 1904
Thu Jan  1 02:53:32 1903
Tue Dec 31 20:53:32 1901
Fri Dec 13 17:39:24 1901
Fri Dec 13 17:39:24 1901
...

On the other hand, this program

	for y=1960,1980 do
		print(y,os.time{year=y,month=1,day=1})
	end

cannot handle years before 1970 (see below). So it seems a limitation
of localtime (which is what os.date uses) not of mktime (which is what
os.time uses). [I wonder why mktime does not give an error past 1902.]

1960	nil
1961	nil
1962	nil
1963	nil
1964	nil
1965	nil
1966	nil
1967	nil
1968	nil
1969	nil
1970	54000
1971	31590000
1972	63126000
1973	94748400
1974	126284400
1975	157820400
1976	189356400
1977	220978800
1978	252514800
1979	284050800
1980	315586800

Oh, the misteries of libc...
--lhf