Synchronize v2.x With Changes from v1.x (#3581)

* docs(tasks): v1.x inventory, triage template, merge report; update task checklist

- Record merge-base and export v1-only commits (v2..v1)
- Document failed merge attempt: v2 file tree diverges from v1
- Mark completed checklist items 0.x, 1.x, merge report draft (5.2)

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* fix(pattern): %z with UTC pattern time shows +00:00 (port v1 09a674b7)

- z_formatter takes pattern_time_type; emit +00:00 when pattern uses UTC
- Track port in tasks/commits-ported.txt and merge-report; update checklist

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* fix(os): Windows utc_minutes_offset via mktime/_mkgmtime (port v1 b656d1ce)

- Replace GetTimeZoneInformation-based offset with v1.x mktime/_mkgmtime approach
- Add tests/test_timezone.cpp and extend pattern_formatter tests; update os.h comment

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* test(timezone): POSIX TZ with DST rules; include fcntl in tcp_client_unix (v1 ports)

- 0f7562a0: EST5EDT / IST-2IDT macros for POSIX vs Windows
- d2100d5d: fcntl.h for Unix tcp client header (v2 path)

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* feat(tcp): connect timeouts and socket IO timeouts; ci: checkout@v6 (v1 ports)

- 9ecdf5c8: connect_socket_with_timeout on Unix/Windows; tcp_sink timeout_ms
- 3c61b051: actions/checkout@v6 on all workflows
- tcp_sink: remove duplicate #pragma once

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* feat(dup_filter_sink): ctor with sink list (v1 45b67eee); add full v1 SHA triage table

- tasks/v1-triage-complete.md: 245 commits PORTED/PENDING/N/A for PRD 3A tracking
- Merge report and task checklist updated

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* feat(level): case-insensitive level_from_str; cmake: BUILD_TYPE only if top-level (v1 ports)

- 566b2d14: common.cpp + test_misc (SUPERSEDED note for d5af52d9 in triage)
- dd3ca04a: guard default CMAKE_BUILD_TYPE for add_subdirectory consumers
- Update v1-triage-complete.md counts and statuses

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* v1 parity: MSVC UTF-8, ansicolor/syslog/os, getenv, triage docs

- CMake: SPDLOG_MSVC_UTF8 + /utf-8 for real MSVC only
- ansicolor: protected target_file_; fix set_color_mode_ lock nesting
- os: drop redundant fileapi.h; spdlog::details::os::getenv via std::getenv
- tests: stopwatch waits 500ms; includes/triage/commits-ported/merge-report updates
- Reclassify superseded v1 SHAs (Sep, syslog, stopwatch ms, utf8 tests, etc.)

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* parity: ringbuffer zero capacity, utf8 assert; v1 triage sync

- ringbuffer_sink: throw if n_items==0 (#3436); test expects spdlog_ex
- utf8_to_wstrbuf: assert compares int to static_cast<int>(target.size()) (#3479)
- Triage: PORTED ad725d34 getenv, 677a2d93 stopwatch, 3f7e5028, a6215527;
  SUPERSEDED a45c9390, eeb22c13, 5673e9e5, fe4f9952, 287333ee
- tasks: 27 PORTED / 26 SUPERSEDED / 78 PENDING; merge-report + commits-ported

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* docs(tasks): note regular commits and push after parity ports

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* parity: dup_filter_sink notification level from last duplicate (#3390)

- Remove notification_level ctor arg; track skipped_msg_log_level_ on duplicate skips
- Test: skipped summary line uses same short level as duplicated messages
- Triage: 847db337 PORTED; 28/26/77 counts; merge-report + commits-ported

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* parity: UWP getenv (WINAPI_FAMILY); triage fmt/cfg/no-except

- os_windows: detect non-desktop/UWP for empty getenv (#3489)
- Triage: PORTED 8806ca65; SUPERSEDED e3f5a4fe e655dbb6; N/A ae1de0dc 548b2642
- Counts 29/28/116/72; merge-report + commits-ported

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* parity: qt_sinks sign casts (#3487); triage 9c582574 superseded

- qt_color_sink: qsizetype for UTF-8 color range lengths; index colors_ with size_t
- SUPERSEDED: 9c582574 — os_unix utc_minutes_offset already matches #3366
- Counts 30/29/70; merge-report + commits-ported

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* parity: SPDLOG_NO_TZ_OFFSET (#3483); triage #3360 superseded

- CMake: option SPDLOG_NO_TZ_OFFSET; PUBLIC compile definition when ON
- z_formatter: +??:?? when macro; else keep UTC +00:00 and local offset
- utc_minutes_offset: stub on Unix/Windows when macro
- test_pattern_formatter: %z UTC case matches placeholder when NO_TZ
- SUPERSEDED 10320184 (ScopedPadder / %D already in v2)
- Triage 31/30/68; merge-report + commits-ported

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* triage: supersede 5931a3d6 ba508057 47b7e7c7 (already on v2 tree)

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* tasks: sync 5.1 triage counts (33 superseded, 65 pending)

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* parity: MSVC/clang hygiene #3515–#3519 #3521; triage batch

- dup_filter_sink: filter_ const, const filter_duration (#3515)
- logger: should_flush uses flush_level() (#3516)
- daily_file_sink: new_filename locals (#3516)
- spdlog::should_log(level log_level) (#3519)
- example my_type value_ / ctor param (#3521)
- Triage: PORTED 1774e700 309204d5 f2a9dec0 472945ba; N/A d299603e 57505989;
  SUPERSEDED 1ef8d3ce 8cfd4a7e; fix ba508057 row

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* parity: udp_sink const udp_sink_config& (#3520); triage fc7e9c87 1685e694

- dist_sink already used std::move(sinks)
- SUPERSEDED: no common-inl.h on v2; log_with_format_ avoids fmt copy path (#3541)

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* parity: README fmt::format_to (#3259); triage 2670f47d d276069a 951c5b99

- Document ambiguous format_to fix in README user-defined type example
- SUPERSEDED: z_formatter warning, fmt11 const formatter, rotate_now + test

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* parity: lock rotate_now mutex (#3281); triage a2b42620 f355b3d5 d276069a

- rotating_file_sink::rotate_now matches #3281 (sync with sink_it_)
- SUPERSEDED: CMake 3.10..3.21 (v2 uses 3.23); daily test fmt::format; fmt11 const row

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* parity: basic_file_sink::truncate (#3280); triage fwrite/fmt/test batch

- truncate(): lock + file_helper::reopen(true)
- test basic_file_sink_truncate
- SUPERSEDED: b7e0e2c2 71925ca3 fa6605dc 885b5473 96c9a62b 1e6250e1 d7155530

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* parity: test_sink/callback iterator cast (#3315); triage Catch2 #3038

- difference_type cast for formatted.end() - eol_len (ad0f31c0)
- SUPERSEDED: c1569a3d Catch2 v3.5.0, 73e2e02b wstr_to_utf8buf bounds

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* parity: SPDLOG_WCHAR_CONSOLE WriteConsoleW path (#3092); triage b6da5944

- WIN32 option SPDLOG_WCHAR_CONSOLE; SPDLOG_UTF8_TO_WCHAR_CONSOLE in private defs
- wincolor print_range_: utf8_to_wstrbuf + WriteConsoleW when defined
- N/A: async_msg flush_callback move-assign (no v1 shape on v2)

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* triage: v1 async_logger/thread_pool + fmt 11.1 batch (15 SHAs)

- N/A: fe79bfcc 6725584e async tests 16e0d2e7 63d18842 d8e0ad46 1e7d7e07 3c23c27d
- SUPERSEDED: faa0a7a9 85bdab0c 276ee5f5 7f8060d5 96a8f625
- Counts: 58 SUPERSEDED, 129 N/A, 17 PENDING; merge-report subsection

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* Port v1 TSAN CMake (#3237); close 3A triage (MDC N/A, fmt 5A)

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* docs(triage): note 3A table complete

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* 5A: bundle fmt 12.1.0 (match v1.x), FMT_INSTALL, MSVC /wd4834, find_dependency(fmt 12)

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* docs(5.4): migration and release notes for v1 parity integration branch

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* docs: cross-link migration notes; refresh PRD/tasks/merge-report for completed 5A/3A

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* revert: remove README and PRD cross-links to migration notes

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* docs(merge-report): audit — add 9fe79692 to ports table; fix 6004e3d1 paths

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* Remove tasks

* PR comment fixes

* Fix small random issues

* Fix PR comments - un-remove the comment and allow any fmt library version

---------

Co-authored-by: Jan Moravec <jan.moravec@hidglobal.com>

tests/CMakeLists.txt

@@ -52,6 +52,7 @@ set(SPDLOG_UTESTS_SOURCES
     test_log_level.cpp
     test_include_sinks.cpp
     test_bin_to_hex.cpp
+    test_timezone.cpp
     test_errors.cpp)
 
 if(WIN32)

tests/includes.h

@@ -25,6 +25,9 @@
 
 #define SPDLOG_ACTIVE_LEVEL SPDLOG_LEVEL_DEBUG
 
+#undef SPDLOG_LEVEL_NAMES
+#undef SPDLOG_SHORT_LEVEL_NAMES
+
 #include "spdlog/details/fmt_helper.h"
 #include "spdlog/pattern_formatter.h"
 #include "spdlog/sinks/null_sink.h"

tests/test_custom_callbacks.cpp

@@ -3,6 +3,9 @@
  * https://raw.githubusercontent.com/gabime/spdlog/v2.x/LICENSE
  */
 #include "includes.h"
+
+#include <iterator>
+
 #include "spdlog/common.h"
 #include "spdlog/sinks/callback_sink.h"
 #include "test_sink.h"
@@ -14,7 +17,8 @@ TEST_CASE("custom_callback_logger", "[custom_callback_logger]") {
         spdlog::memory_buf_t formatted;
         formatter.format(msg, formatted);
         auto eol_len = strlen(spdlog::details::os::default_eol);
-        lines.emplace_back(formatted.begin(), formatted.end() - eol_len);
+        using diff_t = typename std::iterator_traits<decltype(formatted.end())>::difference_type;
+        lines.emplace_back(formatted.begin(), formatted.end() - static_cast<diff_t>(eol_len));
     });
     std::shared_ptr<spdlog::sinks::test_sink_st> test_sink(new spdlog::sinks::test_sink_st);
 

tests/test_dup_filter.cpp

@@ -80,3 +80,41 @@ TEST_CASE("dup_filter_test5", "[dup_filter_sink]") {
     REQUIRE(test_sink->msg_counter() == 3);  // skip 2 messages but log the "skipped.." message before message2
     REQUIRE(test_sink->lines()[1] == "Skipped 2 duplicate messages..");
 }
+
+TEST_CASE("dup_filter_skipped_notification_uses_last_duplicate_level", "[dup_filter_sink]") {
+    using spdlog::sinks::dup_filter_sink_st;
+    using spdlog::sinks::test_sink_mt;
+
+    dup_filter_sink_st dup_sink{std::chrono::seconds{5}};
+    auto test_sink = std::make_shared<test_sink_mt>();
+    test_sink->set_pattern("%L");
+    dup_sink.add_sink(test_sink);
+
+    dup_sink.log(spdlog::details::log_msg{"test", spdlog::level::warn, "same"});
+    dup_sink.log(spdlog::details::log_msg{"test", spdlog::level::warn, "same"});
+    dup_sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "diff"});
+
+    REQUIRE(test_sink->lines().size() == 3);
+    REQUIRE(test_sink->lines()[0] == "W");
+    REQUIRE(test_sink->lines()[1] == "W");
+    REQUIRE(test_sink->lines()[2] == "I");
+}
+
+TEST_CASE("dup_filter_skipped_notification_fixed_level", "[dup_filter_sink]") {
+    using spdlog::sinks::dup_filter_sink_st;
+    using spdlog::sinks::test_sink_mt;
+
+    dup_filter_sink_st dup_sink{std::chrono::seconds{5}, spdlog::level::info};
+    auto test_sink = std::make_shared<test_sink_mt>();
+    test_sink->set_pattern("%L");
+    dup_sink.add_sink(test_sink);
+
+    dup_sink.log(spdlog::details::log_msg{"test", spdlog::level::warn, "same"});
+    dup_sink.log(spdlog::details::log_msg{"test", spdlog::level::warn, "same"});
+    dup_sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "diff"});
+
+    REQUIRE(test_sink->lines().size() == 3);
+    REQUIRE(test_sink->lines()[0] == "W");
+    REQUIRE(test_sink->lines()[1] == "I");
+    REQUIRE(test_sink->lines()[2] == "I");
+}

tests/test_file_logging.cpp

@@ -41,6 +41,26 @@ TEST_CASE("flush_on", "[flush_on]") {
                                                                  default_eol, default_eol, default_eol));
 }
 
+TEST_CASE("basic_file_sink_truncate", "[truncate]") {
+    prepare_logdir();
+    const spdlog::filename_t filename = SPDLOG_FILENAME_T(SIMPLE_LOG);
+    const bool truncate = true;
+    const auto sink = std::make_shared<basic_file_sink_mt>(filename, truncate);
+    const auto logger = std::make_shared<spdlog::logger>("simple_file_logger", sink);
+
+    logger->info("Test message {}", 3.14);
+    logger->info("Test message {}", 2.71);
+    logger->flush();
+    REQUIRE(count_lines(SIMPLE_LOG) == 2);
+
+    sink->truncate();
+    REQUIRE(count_lines(SIMPLE_LOG) == 0);
+
+    logger->info("Test message {}", 6.28);
+    logger->flush();
+    REQUIRE(count_lines(SIMPLE_LOG) == 1);
+}
+
 TEST_CASE("rotating_file_logger1", "[rotating_logger]") {
     prepare_logdir();
     size_t max_size = 1024 * 10;

tests/test_misc.cpp

@@ -72,6 +72,17 @@ TEST_CASE("to_level_enum", "[convert_to_level_enum]") {
     REQUIRE(spdlog::level_from_str("critical") == spdlog::level::critical);
     REQUIRE(spdlog::level_from_str("off") == spdlog::level::off);
     REQUIRE(spdlog::level_from_str("null") == spdlog::level::off);
+    REQUIRE(spdlog::level_from_str("TRACE") == spdlog::level::trace);
+    REQUIRE(spdlog::level_from_str("DEBUG") == spdlog::level::debug);
+    REQUIRE(spdlog::level_from_str("INFO") == spdlog::level::info);
+    REQUIRE(spdlog::level_from_str("WARNING") == spdlog::level::warn);
+    REQUIRE(spdlog::level_from_str("WARN") == spdlog::level::warn);
+    REQUIRE(spdlog::level_from_str("ERROR") == spdlog::level::err);
+    REQUIRE(spdlog::level_from_str("ERR") == spdlog::level::err);
+    REQUIRE(spdlog::level_from_str("CRITICAL") == spdlog::level::critical);
+    REQUIRE(spdlog::level_from_str("OFF") == spdlog::level::off);
+    REQUIRE(spdlog::level_from_str("TrAcE") == spdlog::level::trace);
+    REQUIRE(spdlog::level_from_str("DeBuG") == spdlog::level::debug);
 }
 
 TEST_CASE("copy_ctor", "[copy_ctor]") {

tests/test_pattern_formatter.cpp

@@ -3,6 +3,7 @@
 #include "test_sink.h"
 
 #include <chrono>
+#include <regex>
 
 using spdlog::memory_buf_t;
 
@@ -69,13 +70,34 @@ TEST_CASE("date MM/DD/YY ", "[pattern_formatter]") {
     REQUIRE(log_to_str("Some message", "%D %v", spdlog::pattern_time_type::local, "\n") == oss.str());
 }
 
-TEST_CASE("GMT offset ", "[pattern_formatter]") {
+TEST_CASE("%z with UTC pattern time", "[pattern_formatter]") {
     using namespace std::chrono_literals;
     const auto now = std::chrono::system_clock::now();
     const auto yesterday = now - 24h;
 
+#ifndef SPDLOG_NO_TZ_OFFSET
     REQUIRE(log_to_str_with_time(yesterday, "Some message", "%z", spdlog::pattern_time_type::utc, "\n") ==
             "+00:00\n");
+#else
+    REQUIRE(log_to_str_with_time(yesterday, "Some message", "%z", spdlog::pattern_time_type::utc, "\n") ==
+            "+??:??\n");
+#endif
+}
+
+// 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();
+    std::string result =
+        log_to_str_with_time(now, "Some message", "%z", spdlog::pattern_time_type::local, "\n");
+
+#ifndef SPDLOG_NO_TZ_OFFSET
+    // Match format: +HH:MM or -HH:MM
+    std::regex re(R"([+-]\d{2}:[0-5]\d\n)");
+    REQUIRE(std::regex_match(result, re));
+#else
+    REQUIRE(result == "+??:??\n");
+#endif
 }
 
 TEST_CASE("color range test1", "[pattern_formatter]") {

tests/test_ringbuffer_sink.cpp

@@ -57,15 +57,5 @@ TEST_CASE("test_empty", "[ringbuffer_sink]") {
 }
 
 TEST_CASE("test_empty_size", "[ringbuffer_sink]") {
-    const size_t sink_size = 0;
-    auto sink = std::make_shared<spdlog::sinks::ringbuffer_sink_mt>(sink_size);
-    spdlog::logger l("logger", sink);
-
-    for (size_t i = 0; i < sink_size + 1; ++i) {
-        l.info("{}", i);
-    }
-
-    sink->drain([&](std::string_view) {
-        REQUIRE_FALSE(true);  // should not be called since the sink size is 0
-    });
+    REQUIRE_THROWS_AS((void)spdlog::sinks::ringbuffer_sink_mt(0), spdlog::spdlog_ex);
 }

tests/test_sink.h

@@ -7,6 +7,7 @@
 
 #include <chrono>
 #include <exception>
+#include <iterator>
 #include <mutex>
 #include <thread>
 
@@ -57,7 +58,8 @@ protected:
         // save the line without the eol
         auto eol_len = strlen(details::os::default_eol);
         if (lines_.size() < lines_to_save) {
-            lines_.emplace_back(formatted.begin(), formatted.end() - eol_len);
+            using diff_t = typename std::iterator_traits<decltype(formatted.end())>::difference_type;
+            lines_.emplace_back(formatted.begin(), formatted.end() - static_cast<diff_t>(eol_len));
         }
         msg_counter_++;
         std::this_thread::sleep_for(delay_);

tests/test_stopwatch.cpp

@@ -5,7 +5,7 @@
 TEST_CASE("stopwatch1", "[stopwatch]") {
     using std::chrono::milliseconds;
     using clock = std::chrono::steady_clock;
-    milliseconds wait_ms(200);
+    milliseconds wait_ms(500);
     milliseconds tolerance_ms(250);
     auto start = clock::now();
     spdlog::stopwatch sw;
@@ -22,7 +22,7 @@ TEST_CASE("stopwatch2", "[stopwatch]") {
     using std::chrono::milliseconds;
     using clock = std::chrono::steady_clock;
 
-    clock::duration wait_duration(milliseconds(200));
+    clock::duration wait_duration(milliseconds(500));
     clock::duration tolerance_duration(milliseconds(250));
 
     auto test_sink = std::make_shared<test_sink_st>();

tests/test_timezone.cpp

@@ -0,0 +1,194 @@
+#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;
+
+/*
+ * POSIX 2024 defines three formats for the TZ environment variable,
+ *
+ * 1. Implementation defined format which always starts with a colon:
+ *    ":characters".
+ * 2. A specifier which fully describes the timezone rule in format
+ *    "stdoffset[dst[offset][,start[/time],end[/time]]]". Note the
+ *    offset and start/end part could be omitted, in which case one hour
+ *    is implied, or it's considered implementation-defined when changing
+ *    to and from Daylight Saving Time occurs.
+ * 3. Geographical or special timezone from an implementation-defined
+ *    timezone database.
+ *
+ * On POSIX-compilant systems, we prefer format 2, and explicitly specify the
+ * DST rules to avoid implementation-defined behavior.
+ *
+ * See also IEEE 1003.1-2024 8.3 Other Environment Variables.
+ */
+#ifndef _WIN32
+/*
+ * Standard time is UTC-5 ("EST"), DST time is UTC-4 ("EDT"). DST is active
+ * from 2:00 on the 2nd Sunday in March, to 2:00 on 1st Sunday in November.
+ */
+#define EST5EDT "EST5EDT,M3.2.0,M11.1.0"
+/*
+ * Standard time is UTC+2 ("IST"), DST time is UTC+3 ("IDT"). DST is active
+ * from 2:00 on following day of the 4th Thursday in March, to 2:00 on the
+ * last Sunday in October.
+ */
+#define IST_MINUS2_IDT "IST-2IDT,M3.4.4/26,M10.5.0"
+#else
+/*
+ * However, Windows doesn't follow the POSIX rules and only accept a TZ
+ * environment variable in format
+ *
+ *   tzn [+|-]hh[:mm[:ss] ][dzn]
+ *
+ * thus we couldn't specify the DST rules. Luckily, Windows C runtime library
+ * assumes the United State's rules for implementing the calculation of DST,
+ * which is fine for our test cases.
+ *
+ * See also https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset?view=msvc-170
+ */
+#define EST5EDT "EST5EDT"
+#define IST_MINUS2_IDT "IST-2IDT"
+#endif
+
+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_MINUS2_IDT);
+
+    // 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
+