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

vlib/time/time.v

@@ -401,3 +401,51 @@ pub fn offset() int {
 	local := t.local()
 	return int(local.unix - t.unix)
 }
+
+// local_to_utc converts the receiver `t` to the corresponding UTC time, if it contains local time.
+// If the receiver already does contain UTC time, it returns it unchanged.
+pub fn (t Time) local_to_utc() Time {
+	if !t.is_local {
+		return t
+	}
+	return Time{
+		...t.add(-offset() * time.second)
+		is_local: false
+	}
+}
+
+// utc_to_local converts the receiver `u` to the corresponding local time, if it contains UTC time.
+// If the receiver already does contain local time, it returns it unchanged.
+pub fn (u Time) utc_to_local() Time {
+	if u.is_local {
+		return u
+	}
+	return Time{
+		...u.add(offset() * time.second)
+		is_local: true
+	}
+}
+
+// as_local returns the exact same time, as the receiver `t`, but with its .is_local field set to true.
+// See also #Time.utc_to_local .
+pub fn (t Time) as_local() Time {
+	return Time{
+		...t
+		is_local: true
+	}
+}
+
+// as_utc returns the exact same time, as the receiver `t`, but with its .is_local field set to false.
+// See also #Time.local_to_utc .
+pub fn (t Time) as_utc() Time {
+	return Time{
+		...t
+		is_local: false
+	}
+}
+
+// is_utc returns true, when the receiver `t` is a UTC time, and false otherwise.
+// See also #Time.utc_to_local .
+pub fn (t Time) is_utc() bool {
+	return !t.is_local
+}