Fix incorrect to_gmt_offset result

lib/std/time/datetime.c3

@@ -105,7 +105,8 @@ fn TzDateTime DateTime.to_gmt_offset(self, int gmt_offset)
  * @ensure self.time == return.time
  **/
 fn TzDateTime TzDateTime.to_gmt_offset(self, int gmt_offset) {
-	Time time = self.time + (Time)(gmt_offset - self.gmt_offset) * (Time)time::SEC;
+	if (self.gmt_offset == gmt_offset) return self;
+	Time time = self.time + (Time)gmt_offset * (Time)time::SEC;
 	DateTime dt = from_time(time);
 	dt.time = self.time;
 	return { dt, gmt_offset };

test/unit/stdlib/time/datetime.c3

@@ -49,6 +49,9 @@ fn void test_timezone()
 	int offset_hours = 7;
 	int offset = offset_hours * 3600;
 
+	int target_offset_hours = -4;
+	int target_offset = target_offset_hours * 3600;
+
 	DateTime d1 = datetime::from_date(2022, OCTOBER, 15);
 	TzDateTime d2 = datetime::from_date_tz(2022, OCTOBER, 15, gmt_offset: offset);
 
@@ -127,4 +130,22 @@ fn void test_timezone()
 	assert(tz.day == 15);
 	assert(tz.hour == 0);
 	assert(tz.gmt_offset == offset);
+
+	// Conversion from GMT+7 to GMT-4
+	tz = d2.to_gmt_offset(target_offset);
+	assert(tz.year == d2.year);
+	assert(tz.month == d2.month);
+	assert(tz.day == d2.day - 1);
+	assert(tz.hour == 24 - (offset_hours - target_offset_hours));
+	assert(tz.time == t2);
+	assert(tz.gmt_offset == target_offset);
+
+	// Conversion with the same offset
+	tz = d2.to_gmt_offset(offset);
+	assert(tz.year == d2.year);
+	assert(tz.month == d2.month);
+	assert(tz.day == d2.day);
+	assert(tz.hour == d2.hour);
+	assert(tz.time == t2);
+	assert(tz.gmt_offset == offset);
 }