time_zone: Remove string ops for determing zone

MinGW's strftime implementation does not work and cannot be used to
determine the time zone. Besides that, the string operations are
actually unnecessary since we can get the offset from
std::localtime.

Compare localtime to gmtime to find the zone offset on all platforms.

src/common/time_zone.cpp

@@ -33,32 +33,27 @@ std::string GetDefaultTimeZone() {
     return "GMT";
 }
 
-static std::string GetOsTimeZoneOffset() {
-    const std::time_t t{std::time(nullptr)};
-    const std::tm tm{*std::localtime(&t)};
-
-    return fmt::format("{:%z}", tm);
-}
-
-static int ConvertOsTimeZoneOffsetToInt(const std::string& timezone) {
-    try {
-        return std::stoi(timezone);
-    } catch (const std::invalid_argument&) {
-        LOG_CRITICAL(Common, "invalid_argument with {}!", timezone);
-        return 0;
-    } catch (const std::out_of_range&) {
-        LOG_CRITICAL(Common, "out_of_range with {}!", timezone);
-        return 0;
-    }
+// Results are not comparable to seconds since Epoch
+static std::time_t TmSpecToSeconds(const struct std::tm& spec) {
+    std::time_t cumulative = spec.tm_year;
+    cumulative = cumulative * 365 + spec.tm_yday; // Years to days
+    cumulative = cumulative * 24 + spec.tm_hour;  // Days to hours
+    cumulative = cumulative * 60 + spec.tm_min;   // Hours to minutes
+    cumulative = cumulative * 60 + spec.tm_sec;   // Minutes to seconds
+    return cumulative;
 }
 
 std::chrono::seconds GetCurrentOffsetSeconds() {
-    const int offset{ConvertOsTimeZoneOffsetToInt(GetOsTimeZoneOffset())};
+    const std::time_t t{std::time(nullptr)};
+    const std::tm local{*std::localtime(&t)};
+    const std::tm gmt{*std::gmtime(&t)};
 
-    int seconds{(offset / 100) * 60 * 60}; // Convert hour component to seconds
-    seconds += (offset % 100) * 60;        // Convert minute component to seconds
+    // gmt_seconds is a different offset than time(nullptr)
+    const auto gmt_seconds = TmSpecToSeconds(gmt);
+    const auto local_seconds = TmSpecToSeconds(local);
+    const auto seconds_offset = gmt_seconds - local_seconds;
 
-    return std::chrono::seconds{seconds};
+    return std::chrono::seconds{seconds_offset};
 }
 
 // Key is [Hours * 100 + Minutes], multiplied by 100 if DST
@@ -71,11 +66,6 @@ const static std::map<s64, const char*> off_timezones = {
 };
 
 std::string FindSystemTimeZone() {
-#if defined(MINGW)
-    // MinGW has broken strftime -- https://sourceforge.net/p/mingw-w64/bugs/793/
-    // e.g. fmt::format("{:%z}") -- returns "Eastern Daylight Time" when it should be "-0400"
-    return timezones[0];
-#else
     const s64 seconds = static_cast<s64>(GetCurrentOffsetSeconds().count());
 
     const s64 minutes = seconds / 60;
@@ -97,7 +87,6 @@ std::string FindSystemTimeZone() {
         }
     }
     return fmt::format("Etc/GMT{:s}{:d}", hours > 0 ? "-" : "+", std::abs(hours));
-#endif
 }
 
 } // namespace Common::TimeZone