Fix crash when stringifying pre 1970 dates on Windows

`gmtime*` on Windows fails on dates pre 1970, and because we didn't
check the return code, we would then pass invalid `tm` struct to
`strftime` causing it to assert.

Closes #2944

src/catch2/catch_tostring.hpp

@@ -634,7 +634,11 @@ struct ratio_string<std::milli> {
 
 #ifdef _MSC_VER
             std::tm timeInfo = {};
-            gmtime_s(&timeInfo, &converted);
+            const auto err = gmtime_s(&timeInfo, &converted);
+            if ( err ) {
+                return "gmtime from provided timepoint has failed. This "
+                       "happens e.g. with pre-1970 dates using Microsoft libc";
+            }
 #else
             std::tm* timeInfo = std::gmtime(&converted);
 #endif

tests/SelfTest/IntrospectiveTests/ToString.tests.cpp

@@ -7,10 +7,13 @@
 // SPDX-License-Identifier: BSL-1.0
 
 #include <catch2/internal/catch_enum_values_registry.hpp>
+#include <catch2/matchers/catch_matchers_string.hpp>
 #include <catch2/matchers/catch_matchers_vector.hpp>
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/catch_template_test_macros.hpp>
 
+#include <chrono>
+
 enum class EnumClass3 { Value1, Value2, Value3, Value4 };
 
 struct UsesSentinel {
@@ -95,3 +98,23 @@ TEMPLATE_TEST_CASE( "Stringifying char arrays with statically known sizes",
     TestType no_null_terminator[3] = { 'a', 'b', 'c' };
     CHECK( ::Catch::Detail::stringify( no_null_terminator ) == R"("abc")"s );
 }
+
+TEST_CASE( "#2944 - Stringifying dates before 1970 should not crash" ) {
+    using Catch::Matchers::Equals;
+    using Days = std::chrono::duration<int32_t, std::ratio<86400>>;
+    using SysDays = std::chrono::time_point<std::chrono::system_clock, Days>;
+    Catch::StringMaker<std::chrono::system_clock::time_point> sm;
+
+    // Check simple date first
+    const SysDays post1970{ Days{ 1 } };
+    auto converted_post = sm.convert( post1970 );
+    REQUIRE( converted_post == "1970-01-02T00:00:00Z" );
+
+    const SysDays pre1970{ Days{ -1 } };
+    auto converted_pre = sm.convert( pre1970 );
+    REQUIRE_THAT(
+        converted_pre,
+        Equals( "1969-12-31T00:00:00Z" ) ||
+            Equals( "gmtime from provided timepoint has failed. This "
+                    "happens e.g. with pre-1970 dates using Microsoft libc" ) );
+}