1 files changed, 71 insertions, 4 deletions

diff --git a/src/common/time_zone.cpp b/src/common/time_zone.cpp
index 126836b01..717d751ba 100644
--- a/src/common/time_zone.cpp
+++ b/src/common/time_zone.cpp
@@ -4,12 +4,29 @@
 #include <chrono>
 #include <iomanip>
 #include <sstream>
+#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 +35,7 @@ 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 +59,57 @@ std::chrono::seconds GetCurrentOffsetSeconds() {
     return std::chrono::seconds{seconds};
 }
 
+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
+    // Time zone offset in seconds from GMT
+    constexpr std::array offsets{
+        0,     0,     3600,  -21600, -19768, 7200,   7509,   -1521, -18000, -18000, -75,    -75,
+        0,     0,     0,     0,      0,      27402,  -36000, -968,  12344,  8454,   -18430, 33539,
+        40160, 3164,  3600,  -25200, -25200, -25196, 41944,  44028, 5040,   -2205,  29143,  -28800,
+        29160, 30472, 24925, 6952,   0,      0,      0,      9017,  0,      0,
+    };
+
+    // If the time zone recognizes Daylight Savings Time
+    constexpr std::array dst{
+        false, false, true,  true,  true,  true,  true,  true,  false, true,  true, true,
+        false, false, false, false, false, true,  false, false, true,  true,  true, true,
+        false, true,  true,  false, true,  true,  true,  true,  true,  true,  true, true,
+        true,  true,  true,  true,  false, false, false, true,  true,  false,
+    };
+
+    static std::string system_time_zone_cached{};
+    if (!system_time_zone_cached.empty()) {
+        return system_time_zone_cached;
+    }
+
+    const auto now = std::time(nullptr);
+    const struct std::tm& local = *std::localtime(&now);
+
+    const s64 system_offset = GetCurrentOffsetSeconds().count() - (local.tm_isdst ? 3600 : 0);
+
+    int min = std::numeric_limits<int>::max();
+    int min_index = -1;
+    for (u32 i = 2; i < offsets.size(); i++) {
+        // Skip if system is celebrating DST but considered time zone does not
+        if (local.tm_isdst && !dst[i]) {
+            continue;
+        }
+
+        const auto offset = offsets[i];
+        const int difference = static_cast<int>(std::abs(offset - system_offset));
+        if (difference < min) {
+            min = difference;
+            min_index = i;
+        }
+    }
+
+    system_time_zone_cached = GetTimeZoneStrings()[min_index];
+    return system_time_zone_cached;
+#endif
+}
+
 } // namespace Common::TimeZone