1 files changed, 59 insertions, 4 deletions

diff --git a/src/common/time_zone.cpp b/src/common/time_zone.cpp
index 126836b01..d8d7896c6 100644
--- a/src/common/time_zone.cpp
+++ b/src/common/time_zone.cpp
@@ -2,14 +2,33 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #include <chrono>
+#include <exception>
 #include <iomanip>
 #include <sstream>
+#include <stdexcept>
+#include <fmt/chrono.h>
+#include <fmt/core.h>
 
 #include "common/logging/log.h"
+#include "common/settings.h"
 #include "common/time_zone.h"
 
 namespace Common::TimeZone {
 
+// Time zone strings
+constexpr std::array timezones{
+    "GMT",       "GMT",       "CET", "CST6CDT", "Cuba",    "EET",    "Egypt",     "Eire",
+    "EST",       "EST5EDT",   "GB",  "GB-Eire", "GMT",     "GMT+0",  "GMT-0",     "GMT0",
+    "Greenwich", "Hongkong",  "HST", "Iceland", "Iran",    "Israel", "Jamaica",   "Japan",
+    "Kwajalein", "Libya",     "MET", "MST",     "MST7MDT", "Navajo", "NZ",        "NZ-CHAT",
+    "Poland",    "Portugal",  "PRC", "PST8PDT", "ROC",     "ROK",    "Singapore", "Turkey",
+    "UCT",       "Universal", "UTC", "W-SU",    "WET",     "Zulu",
+};
+
+const std::array<const char*, 46>& GetTimeZoneStrings() {
+    return timezones;
+}
+
 std::string GetDefaultTimeZone() {
     return "GMT";
 }
@@ -18,10 +37,7 @@ static std::string GetOsTimeZoneOffset() {
     const std::time_t t{std::time(nullptr)};
     const std::tm tm{*std::localtime(&t)};
 
-    std::stringstream ss;
-    ss << std::put_time(&tm, "%z"); // Get the current timezone offset, e.g. "-400", as a string
-
-    return ss.str();
+    return fmt::format("{:%z}", tm);
 }
 
 static int ConvertOsTimeZoneOffsetToInt(const std::string& timezone) {
@@ -45,4 +61,43 @@ std::chrono::seconds GetCurrentOffsetSeconds() {
     return std::chrono::seconds{seconds};
 }
 
+// Key is [Hours * 100 + Minutes], multiplied by 100 if DST
+const static std::map<s64, const char*> off_timezones = {
+    {530, "Asia/Calcutta"},          {930, "Australia/Darwin"},     {845, "Australia/Eucla"},
+    {103000, "Australia/Adelaide"},  {1030, "Australia/Lord_Howe"}, {630, "Indian/Cocos"},
+    {1245, "Pacific/Chatham"},       {134500, "Pacific/Chatham"},   {-330, "Canada/Newfoundland"},
+    {-23000, "Canada/Newfoundland"}, {430, "Asia/Kabul"},           {330, "Asia/Tehran"},
+    {43000, "Asia/Tehran"},          {545, "Asia/Kathmandu"},       {-930, "Asia/Marquesas"},
+};
+
+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;
+    const s64 hours = minutes / 60;
+
+    const s64 minutes_off = minutes - hours * 60;
+
+    if (minutes_off != 0) {
+        const auto the_time = std::time(nullptr);
+        const struct std::tm& local = *std::localtime(&the_time);
+        const bool is_dst = local.tm_isdst != 0;
+
+        const s64 tz_index = (hours * 100 + minutes_off) * (is_dst ? 100 : 1);
+
+        try {
+            return off_timezones.at(tz_index);
+        } catch (std::out_of_range&) {
+            LOG_ERROR(Common, "Time zone {} not handled, defaulting to hour offset.", tz_index);
+        }
+    }
+    return fmt::format("Etc/GMT{:s}{:d}", hours > 0 ? "-" : "+", std::abs(hours));
+#endif
+}
+
 } // namespace Common::TimeZone