The `timegm()` function performs the inverse operation of `gmtime()`
by taking a broken-down date and time dictionary and transforming it into
an epoch value, assuming UTC time.

-- Testcase --
{%
	// check expected epoch
	let d1 = {
		"sec": 42,
		"min": 51,
		"hour": 13,
		"mday": 22,
		"mon": 3,
		"year": 2022,
		"wday": 2,
		"yday": 81,
		"isdst": 0
	};

	// check that out of range values are normalized
	let d2 = {
		"sec": 33,
		"min": 22,
		"hour": 11,
		"mday": 40,
		"mon": 10,
		"year": 2022,
		"wday": 2,
		"yday": 81,
		"isdst": 0
	};

	// check that everything except mday, mon, year is optional
	let d3 = {
		"mday": 1,
		"mon": 1,
		"year": 2000
	};

	printf("%.J\n", [
		timegm(d1),
		timegm(d2),
		timegm(d3)
	]);
%}
-- End --

-- Expect stdout --
[
	1647957102,
	1667992953,
	946684800
]
-- End --