time: add a tm_gmtoff field to C.tm (a BSD and GNU extension) (#20907)

vlib/time/time_nix.c.v

@@ -8,15 +8,16 @@ module time
 #include <errno.h>
 
 pub struct C.tm {
-	tm_sec   int
-	tm_min   int
-	tm_hour  int
-	tm_mday  int
-	tm_mon   int
-	tm_year  int
-	tm_wday  int
-	tm_yday  int
-	tm_isdst int
+	tm_sec    int
+	tm_min    int
+	tm_hour   int
+	tm_mday   int
+	tm_mon    int
+	tm_year   int
+	tm_wday   int
+	tm_yday   int
+	tm_isdst  int
+	tm_gmtoff int
 }
 
 fn C.timegm(&C.tm) C.time_t

vlib/time/time_test.v

@@ -344,3 +344,16 @@ fn test_parse_three_letters_month() {
 	tm_tm := time.parse_format(tm_s, format)!
 	assert tm_tm.month == tm.month
 }
+
+fn test_tm_gmtoff() {
+	$if windows {
+		return
+	} $else {
+		rawtime := i64(0) // C.time_t{}
+
+		C.time(&rawtime) // C.tm{}
+
+		info := C.localtime(&rawtime)
+		assert info.tm_gmtoff == time.now().unix - time.utc().unix
+	}
+}