Test tm timezone formatting

Author: vitaut

test/chrono-test.cc

@@ -336,9 +336,24 @@ TEST(chrono_test, local_time) {
                    fmt::format_error, "no timezone");
 }
 
+template <typename T, FMT_ENABLE_IF(fmt::detail::has_member_data_tm_gmtoff<T>::value)>
+bool set_gmtoff(T& time, long offset) {
+  time.tm_gmtoff = offset;
+  return true;
+}
+template <typename T, FMT_ENABLE_IF(!fmt::detail::has_member_data_tm_gmtoff<T>::value)>
+bool set_gmtoff(T&, long) {
+  return false;
+}
+
 TEST(chrono_test, tm) {
   auto time = fmt::gmtime(290088000);
   test_time(time);
+  if (set_gmtoff(time, -28800)) {
+    EXPECT_EQ(fmt::format(fmt::runtime("{:%z}"), time), "-0800");
+    EXPECT_EQ(fmt::format(fmt::runtime("{:%Ez}"), time), "-08:00");
+    EXPECT_EQ(fmt::format(fmt::runtime("{:%Oz}"), time), "-08:00");
+  }
 }
 
 TEST(chrono_test, daylight_savings_time_end) {