Windows utc_minutes_offset(): Fix historical DST accuracy and improve offset calculation speed (~2.5x) (#3508)

* New utc offset impl for windows and unit tests

* Update utc_minutes_offset()

* Fix warning

* Fix warning

* Fix timezone tests

* Fix timezone tests

* Update tests/test_pattern_formatter.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update tests/CMakeLists.txt

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Updated utc_minutes_offset() impl

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

include/spdlog/details/os-inl.h

@@ -243,31 +243,29 @@ SPDLOG_INLINE size_t filesize(FILE *f) {
 #pragma warning(pop)
 #endif
 
-// Return utc offset in minutes or throw spdlog_ex on failure
 #if !defined(SPDLOG_NO_TZ_OFFSET)
-SPDLOG_INLINE int utc_minutes_offset(const std::tm &tm) {
 #ifdef _WIN32
-#if _WIN32_WINNT < _WIN32_WINNT_WS08
-    TIME_ZONE_INFORMATION tzinfo;
-    auto rv = ::GetTimeZoneInformation(&tzinfo);
-#else
-    DYNAMIC_TIME_ZONE_INFORMATION tzinfo;
-    auto rv = ::GetDynamicTimeZoneInformation(&tzinfo);
-#endif
-    if (rv == TIME_ZONE_ID_INVALID) throw_spdlog_ex("Failed getting timezone info. ", errno);
-
-    int offset = -tzinfo.Bias;
-    if (tm.tm_isdst) {
-        offset -= tzinfo.DaylightBias;
-    } else {
-        offset -= tzinfo.StandardBias;
+// Compare the timestamp as Local (mktime) vs UTC (_mkgmtime) to get the offset.
+SPDLOG_INLINE int utc_minutes_offset(const std::tm &tm) {
+    std::tm local_tm = tm;  // copy since mktime might adjust it (normalize dates, set tm_isdst)
+    std::time_t local_time_t = std::mktime(&local_tm);
+    if (local_time_t == -1) {
+        return 0; // fallback
     }
-    return offset;
-#else
-    auto offset_seconds = tm.tm_gmtoff;
+
+    std::time_t utc_time_t = _mkgmtime(&local_tm);
+    if (utc_time_t == -1) {
+        return 0; // fallback
+    }
+    auto offset_seconds = utc_time_t - local_time_t;
     return static_cast<int>(offset_seconds / 60);
-#endif
 }
+#else
+// On unix simply use tm_gmtoff
+SPDLOG_INLINE int utc_minutes_offset(const std::tm &tm) {
+    return static_cast<int>(tm.tm_gmtoff / 60);
+}
+#endif  // _WIN32
 #endif  // SPDLOG_NO_TZ_OFFSET
 
 // Return current thread id as size_t

tests/CMakeLists.txt

@@ -50,7 +50,9 @@ set(SPDLOG_UTESTS_SOURCES
     test_stopwatch.cpp
     test_circular_q.cpp
     test_bin_to_hex.cpp
-    test_ringbuffer.cpp)
+    test_ringbuffer.cpp
+    test_timezone.cpp
+)
 
 if(NOT SPDLOG_NO_EXCEPTIONS)
     list(APPEND SPDLOG_UTESTS_SOURCES test_errors.cpp)

tests/test_pattern_formatter.cpp

@@ -1,6 +1,6 @@
 #include "includes.h"
 #include "test_sink.h"
-
+#include <regex>
 #include <chrono>
 
 using spdlog::memory_buf_t;
@@ -77,18 +77,20 @@ TEST_CASE("date MM/DD/YY ", "[pattern_formatter]") {
             oss.str());
 }
 
-TEST_CASE("GMT offset ", "[pattern_formatter]") {
+// see test_timezone.cpp for actual UTC offset calculation tests
+TEST_CASE("UTC offset", "[pattern_formatter]") {
     using namespace std::chrono_literals;
     const auto now = std::chrono::system_clock::now();
-    const auto yesterday = now - 24h;
+    std::string result =
+        log_to_str_with_time(now, "Some message", "%z", spdlog::pattern_time_type::local, "\n");
 
 #ifndef SPDLOG_NO_TZ_OFFSET
-    const std::string expected_result = "+00:00\n";
+    // Match format: +HH:MM or -HH:MM
+    std::regex re(R"([+-]\d{2}:[0-5]\d\n)");
+    REQUIRE(std::regex_match(result, re));
 #else
-    const std::string expected_result = "+??:??\n";
+    REQUIRE(result == "+??:??\n");
 #endif
-    REQUIRE(log_to_str_with_time(yesterday, "Some message", "%z", spdlog::pattern_time_type::utc,
-                                 "\n") == expected_result);
 }
 
 TEST_CASE("color range test1", "[pattern_formatter]") {

tests/test_timezone.cpp

@@ -0,0 +1,146 @@
+#ifndef SPDLOG_NO_TZ_OFFSET
+
+#include "includes.h"
+#include <ctime>
+#include <cstdlib>
+#include <cstring>
+
+// Helper to construct a simple std::tm from components
+std::tm make_tm(int year, int month, int day, int hour, int minute) {
+    std::tm t;
+    std::memset(&t, 0, sizeof(t));
+    t.tm_year = year - 1900;
+    t.tm_mon = month - 1;
+    t.tm_mday = day;
+    t.tm_hour = hour;
+    t.tm_min = minute;
+    t.tm_sec = 0;
+    t.tm_isdst = -1;
+    std::mktime(&t);
+    return t;
+}
+
+// Cross-platform RAII Helper to safely set/restore process timezone
+class ScopedTZ {
+    std::string original_tz_;
+    bool has_original_ = false;
+
+public:
+    explicit ScopedTZ(const std::string& tz_name) {
+        // save current TZ
+#ifdef _WIN32
+        char* buf = nullptr;
+        size_t len = 0;
+        if (_dupenv_s(&buf, &len, "TZ") == 0 && buf != nullptr) {
+            original_tz_ = std::string(buf);
+            has_original_ = true;
+            free(buf);
+        }
+#else
+        const char* tz = std::getenv("TZ");
+        if (tz) {
+            original_tz_ = tz;
+            has_original_ = true;
+        }
+#endif
+
+        // set new TZ
+#ifdef _WIN32
+        _putenv_s("TZ", tz_name.c_str());
+        _tzset();
+#else
+        setenv("TZ", tz_name.c_str(), 1);
+        tzset();
+#endif
+    }
+
+    ~ScopedTZ() {
+        // restore original TZ
+#ifdef _WIN32
+        if (has_original_) {
+            _putenv_s("TZ", original_tz_.c_str());
+        } else {
+            _putenv_s("TZ", "");
+        }
+        _tzset();
+#else
+        if (has_original_) {
+            setenv("TZ", original_tz_.c_str(), 1);
+        } else {
+            unsetenv("TZ");
+        }
+        tzset();
+#endif
+    }
+};
+
+using spdlog::details::os::utc_minutes_offset;
+
+TEST_CASE("UTC Offset - Western Hemisphere (USA - Standard Time)", "[timezone][west]") {
+    // EST5EDT: Eastern Standard Time (UTC-5)
+    ScopedTZ tz("EST5EDT");
+
+    // Jan 15th (Winter)
+    auto tm = make_tm(2023, 1, 15, 12, 0);
+    REQUIRE(utc_minutes_offset(tm) == -300);
+}
+
+TEST_CASE("UTC Offset - Eastern Hemisphere (Europe/Israel - Standard Time)", "[timezone][east]") {
+    // IST-2IDT: Israel Standard Time (UTC+2)
+    ScopedTZ tz("IST-2IDT");
+
+    // Jan 15th (Winter)
+    auto tm = make_tm(2023, 1, 15, 12, 0);
+    REQUIRE(utc_minutes_offset(tm) == 120);
+}
+
+TEST_CASE("UTC Offset - Zero Offset (UTC/GMT)", "[timezone][utc]") {
+    ScopedTZ tz("GMT0");
+
+    // Check Winter
+    auto tm_winter = make_tm(2023, 1, 15, 12, 0);
+    REQUIRE(utc_minutes_offset(tm_winter) == 0);
+
+    // Check Summer (GMT never shifts, so this should also be 0)
+    auto tm_summer = make_tm(2023, 7, 15, 12, 0);
+    REQUIRE(utc_minutes_offset(tm_summer) == 0);
+}
+
+TEST_CASE("UTC Offset - Non-Integer Hour Offsets (India)", "[timezone][partial]") {
+    // IST-5:30: India Standard Time (UTC+5:30)
+    ScopedTZ tz("IST-5:30");
+
+    auto tm = make_tm(2023, 1, 15, 12, 0);
+    REQUIRE(utc_minutes_offset(tm) == 330);
+}
+
+TEST_CASE("UTC Offset - Edge Case: Negative Offset Crossing Midnight", "[timezone][edge]") {
+    ScopedTZ tz("EST5EDT");
+    // Late night Dec 31st, 2023
+    auto tm = make_tm(2023, 12, 31, 23, 59);
+    REQUIRE(utc_minutes_offset(tm) == -300);
+}
+
+TEST_CASE("UTC Offset - Edge Case: Leap Year", "[timezone][edge]") {
+    ScopedTZ tz("EST5EDT");
+    // Feb 29, 2024 (Leap Day) - Winter
+    auto tm = make_tm(2024, 2, 29, 12, 0);
+    REQUIRE(utc_minutes_offset(tm) == -300);
+}
+
+TEST_CASE("UTC Offset - Edge Case: Invalid Date (Pre-Epoch)", "[timezone][edge]") {
+#ifdef _WIN32
+    // Windows mktime returns -1 for dates before 1970.
+    // We expect the function to safely return 0 (fallback).
+    auto tm = make_tm(1960, 1, 1, 12, 0);
+    REQUIRE(utc_minutes_offset(tm) == 0);
+#else
+    // Unix mktime handles pre-1970 dates correctly.
+    // We expect the actual historical offset (EST was UTC-5 in 1960).
+    ScopedTZ tz("EST5EDT");
+    auto tm = make_tm(1960, 1, 1, 12, 0);
+    REQUIRE(utc_minutes_offset(tm) == -300);
+#endif
+}
+
+#endif  // !SPDLOG_NO_TZ_OFFSET