time: fix date parsing tests in winter for regions using DST (#7304)

.github/workflows/ci.yml

@@ -103,6 +103,8 @@ jobs:
       run: TZ=Etc/GMT-3 ./v test vlib/time/
     - name: Test time functions in a timezone UTC+12
       run: TZ=Etc/GMT-12 ./v test vlib/time/
+    - name: Test time functions in a timezone using daylight saving (Europe/Paris)
+      run: TZ=Europe/Paris ./v test vlib/time/
     - name: Test building v tools
       run: ./v -silent build-tools
     - name: v doctor

vlib/time/parse.v

@@ -111,5 +111,5 @@ pub fn parse_iso8601(s string) ?Time {
 		t = unix2(int(unix_time), t.microsecond)
 	}
 	// Convert the time to local time
-	return to_local_time(t)
+	return t.to_local_time()
 }

vlib/time/parse_test.v

@@ -1,5 +1,4 @@
 import time
-import math
 
 fn test_parse() {
 	s := '2018-01-27 12:48:34'
@@ -50,15 +49,20 @@ fn test_parse_rfc2822_invalid() {
 }
 
 fn test_parse_iso8601() {
-	// Because there is a small difference between time.now() - time.utc and actual offset,
-	// round to the nearest hour.
-	offset := time.Duration(i64(math.round((time.now() - time.utc()).hours())) * time.hour)
+	// Because the offset between local time and UTC is not constant in regions
+	// that use daylight saving times we need to calculate separete offsets for
+	// summer and winter
+	offset_summer := time.Duration(time.new_time(year: 2020, month: 6, day: 5, hour: 15).to_local_time() -
+		time.new_time(year: 2020, month: 6, day: 5, hour: 15))
+	offset_winter := time.Duration(time.new_time(year: 2020, month: 11, day: 5, hour: 15).to_local_time() -
+		time.new_time(year: 2020, month: 11, day: 5, hour: 15))
 	formats := [
 		'2020-06-05T15:38:06Z',
 		'2020-06-05T15:38:06.015959Z',
 		'2020-06-05T15:38:06.015959+00:00',
 		'2020-06-05T15:38:06.015959+02:00',
 		'2020-06-05T15:38:06.015959-02:00',
+		'2020-11-05T15:38:06.015959Z'
 	]
 	times := [
 		[2020, 6, 5, 15, 38, 6, 0],
@@ -66,6 +70,7 @@ fn test_parse_iso8601() {
 		[2020, 6, 5, 15, 38, 6, 15959],
 		[2020, 6, 5, 13, 38, 6, 15959],
 		[2020, 6, 5, 17, 38, 6, 15959],
+		[2020, 11, 5, 15, 38, 6, 15959],
 	]
 	for i, format in formats {
 		t := time.parse_iso8601(format) or {
@@ -81,7 +86,7 @@ fn test_parse_iso8601() {
 			minute: data[4]
 			second: data[5]
 			microsecond: data[6]
-		).add(offset)
+		).add(if i <= 4 { offset_summer } else { offset_winter })
 		assert t.year == t2.year
 		assert t.month == t2.month
 		assert t.day == t2.day

vlib/time/time_nix.c.v

@@ -24,7 +24,7 @@ fn make_unix_time(t C.tm) int {
 	return int(C.timegm(&t))
 }
 
-fn to_local_time(t Time) Time {
+pub fn (t Time) to_local_time() Time {
 	loc_tm := C.tm{}
 	C.localtime_r(time_t(&t.unix), &loc_tm)
 	return convert_ctime(loc_tm, t.microsecond)

vlib/time/time_windows.c.v

@@ -93,7 +93,7 @@ fn local_as_unix_time() int {
 	return make_unix_time(tm)
 }
 
-fn to_local_time(t Time) Time {
+pub fn (t Time) to_local_time() Time {
 	st_utc := SystemTime{
 		year: u16(t.year)
 		month: u16(t.month)