lib: add date and time related functions

Add five new functions to deal with date calculation and timing:

 - localtime(), gmtime() - return a broken down calendar date and time
   specification from the given epoch (or now, if absent) in local and
   UTC time respectively

 - timelocal(), timegm() - the inverse operation for the former functions,
   taking a date and time specification (interpreted as local or UTC time
   respectively) and turning it into an epoch value

 - clock() - return the second and nanosecond values of the system clock,
   useful for time/performance measurements

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

1 files changed, 54 insertions, 0 deletions

diff --git a/tests/custom/03_stdlib/54_timegm b/tests/custom/03_stdlib/54_timegm
new file mode 100644
index 0000000..9c0b59a
--- /dev/null
+++ b/tests/custom/03_stdlib/54_timegm
@@ -0,0 +1,54 @@
+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 --