time: add more UTC/local time conversion functions, make Time.format_rfc3339 more stable

vlib/time/utc_vs_local_time_test.v

@@ -0,0 +1,48 @@
+import time
+
+fn test_as_utc() {
+	t := time.now()
+	u := t.as_utc()
+	dump(u)
+	assert u.is_local == false
+}
+
+fn test_as_local() {
+	t := time.now()
+	l := t.as_local()
+	dump(l)
+	assert l.is_local == true
+}
+
+fn test_local_to_utc() {
+	n := time.now()
+	u := n.local_to_utc()
+	dump(u)
+	o := time.offset()
+	dump(o)
+	if o != 0 {
+		assert n != u
+		back := u.utc_to_local() // convert it back to local time
+		assert n == back, 'the converted original local->utc->local time, should be the same as the original local time'
+
+		double_u := u.local_to_utc()
+		assert u == double_u, 'calling t.local_to_utc().local_to_utc() several times, should not change the time'
+	}
+}
+
+fn test_utc_to_local() {
+	z := time.Time{}
+	assert z.is_local == false, 'simply constructing a time instance, should construct an UTC time'
+	l := z.utc_to_local()
+	dump(l)
+	o := time.offset()
+	dump(o)
+	if o != 0 {
+		assert z != l, 'when there is a time offset, the local time and the utc time should be different'
+		assert l == l.utc_to_local(), 'converting a local to local time should not change the time'
+		assert l == l.utc_to_local().utc_to_local(), 'double converting a local to local time to local time, should not change the time'
+	}
+	sz := z.format_rfc3339()
+	dump(sz)
+	assert sz == '0000-00-00T00:00:00.000Z'
+}