Lua FFI implementation of timegm (convert any given date to GMT epoch timestamp)

timegm.lua

local ffi = require 'ffi'

if not pcall(ffi.typeof, 'struct tm') then
	ffi.cdef[[
		struct tm {
			int tm_sec;
			int tm_min;
			int tm_hour;
			int tm_mday;
			int tm_mon;
			int tm_year;
			int tm_wday;
			int tm_yday;
			int tm_isdst;
			long tm_gmtoff;
			char *tm_zone;
		};
	]]
end

if not pcall(function() return ffi.C.timegm end) then
	ffi.cdef[[
		time_t timegm(struct tm *timeptr);
	]]
end

local dst = ffi.new('struct tm') -- zeroed by default

--[[
usage:
timegm( sec, min, hour, day, month, year )

month is month number minus one, i.e. Jan is 0 and Dec is 11
year should be year minus 1900, so 2021 should be passed as 121

]]

local function timegm(s, m, h, D, M, Y)
	dst.tm_sec = s or 0
	dst.tm_min = m or 0
	dst.tm_hour = h or 0
	dst.tm_mday = D or 0
	dst.tm_mon = M or 0
	dst.tm_year = Y or 0
	return tonumber( ffi.C.timegm(dst) )
end

return timegm