diff --git a/.github/workflows/ZenLib_Checks.yml b/.github/workflows/ZenLib_Checks.yml
index ab69b90..1ed58bc 100644
--- a/.github/workflows/ZenLib_Checks.yml
+++ b/.github/workflows/ZenLib_Checks.yml
@@ -63,11 +63,12 @@ jobs:
           - platform: Win32
             msystem: MINGW32
           - platform: x64
-            msystem: MINGW64
+            msystem: UCRT64
       fail-fast: false
     env:
       GENERATOR: "Unix Makefiles"
       CONFIGURATION: "Release"
+      CXXFLAGS: -Werror
     steps:
       - name: Checkout ZenLib
         uses: actions/checkout@v5
diff --git a/Source/ZenLib/Format/Http/Http_Cookies.cpp b/Source/ZenLib/Format/Http/Http_Cookies.cpp
index 207f8ee..4867cd1 100644
--- a/Source/ZenLib/Format/Http/Http_Cookies.cpp
+++ b/Source/ZenLib/Format/Http/Http_Cookies.cpp
@@ -86,20 +86,28 @@ void Cookies::Create_Lines(std::ostream& Out)
         if (Cookie->second.Expires!=(time_t)-1)
         {
             char Temp[200];
-            #if defined(HAVE_GMTIME_R)
-            struct tm Gmt_Temp;
-            struct tm *Gmt=gmtime_r(&Cookie->second.Expires, &Gmt_Temp);
-            #elif defined(_MSC_VER)
-            struct tm Gmt_Temp;
-            errno_t gmtime_s_Result=gmtime_s(&Gmt_Temp , &Cookie->second.Expires);
-            struct tm* Gmt=gmtime_s_Result?NULL:&Gmt_Temp;
+            #if defined(_WIN32)
+                #if _CRT_USE_CONFORMING_ANNEX_K_TIME || (__STDC_WANT_LIB_EXT1__ && defined(__STDC_LIB_EXT1__))
+                // C11 standard version on MSVC or other compilers
+                tm Gmt_Temp;
+                tm* Gmt = gmtime_s(&Cookie->second.Expires, &Gmt_Temp);
+                #else
+                // MSVC and MinGW-w64 argument order and return value differs from C11 standard
+                tm Gmt_Temp;
+                errno_t gmtime_s_Result = gmtime_s(&Gmt_Temp, &Cookie->second.Expires);
+                tm* Gmt = gmtime_s_Result ? nullptr : &Gmt_Temp;
+                #endif
+            #elif defined(HAVE_GMTIME_R)
+                // POSIX or C23
+                tm Gmt_Temp;
+                tm *Gmt = gmtime_r(&Cookie->second.Expires, &Gmt_Temp);
             #else
-            #ifdef __GNUC__
-            #warning "This version of ZenLib is not thread safe"
+                // Fallback: not thread-safe, but prevents compile errors
+                #ifdef __GNUC__
+                #warning "This version of ZenLib is not thread safe"
+                #endif
+                tm *Gmt = gmtime(&Cookie->second.Expires);
             #endif
-            struct tm *Gmt=gmtime(&Cookie->second.Expires);
-            #endif
-
             if (Gmt)
                 if (strftime(Temp, 200, "%a, %d-%b-%Y %H:%M:%S GMT", Gmt))
                     Out << "; expires=" << Temp;
diff --git a/Source/ZenLib/Utils.cpp b/Source/ZenLib/Utils.cpp
index 7f6daf4..4074c70 100644
--- a/Source/ZenLib/Utils.cpp
+++ b/Source/ZenLib/Utils.cpp
@@ -19,6 +19,7 @@
 #include "ZenLib/Utils.h"
 #include <cmath>
 #include <complex>
+#include <limits>
 //---------------------------------------------------------------------------
 
 namespace ZenLib
@@ -972,16 +973,16 @@ void   int64u_int32u (int64u BigInt, int32u &High, int32u &Low)
 int32s float32_int32s (float32 F, bool Rounded)
 {
     //Out of boundaries
-    if (F>=(int32s)0x7FFFFFFF)
-        return (int32s)0x7FFFFFFF;
-    if (F<=(int32s)0x80000000)
-        return (int32s)0x80000000;
+    if (F>=(float32)std::numeric_limits<int32s>::max())
+        return std::numeric_limits<int32s>::max();
+    if (F<=(float32)std::numeric_limits<int32s>::min())
+        return std::numeric_limits<int32s>::min();
 
     //Not rounded
     if (!Rounded)
         return (int32s)F;
     //Rounded
-    int I1=(int)F;
+    int32s I1=(int32s)F;
     if (F-I1>=0.5f)
         return I1+1;
     else
@@ -991,16 +992,16 @@ int32s float32_int32s (float32 F, bool Rounded)
 int64s float32_int64s (float32 F, bool Rounded)
 {
     //Out of boundaries
-    if (F>=(int64s)0x7FFFFFFFFFFFFFFFLL)
-        return (int64s)0x7FFFFFFFFFFFFFFFLL;
-    if (F<=(int64s)0x8000000000000000LL)
-        return (int64s)0x8000000000000000LL;
+    if (F>=(float32)std::numeric_limits<int64s>::max())
+        return std::numeric_limits<int64s>::max();
+    if (F<=(float32)std::numeric_limits<int64s>::min())
+        return std::numeric_limits<int64s>::min();
 
     //Not rounded
     if (!Rounded)
         return (int64s)F;
     //Rounded
-    int I1=(int)F;
+    int64s I1=(int64s)F;
     if (F-I1>=0.5f)
         return I1+1;
     else
@@ -1010,10 +1011,10 @@ int64s float32_int64s (float32 F, bool Rounded)
 int32s float64_int32s (float64 F, bool Rounded)
 {
     //Out of boundaries
-    if (F>=(int32s)0x7FFFFFFF)
-        return (int32s)0x7FFFFFFF;
-    if (F<=(int32s)0x80000000)
-        return (int32s)0x80000000;
+    if (F>=(float64)std::numeric_limits<int32s>::max())
+        return std::numeric_limits<int32s>::max();
+    if (F<=(float64)std::numeric_limits<int32s>::min())
+        return std::numeric_limits<int32s>::min();
 
     //Not rounded
     if (!Rounded)
@@ -1029,10 +1030,10 @@ int32s float64_int32s (float64 F, bool Rounded)
 int64s float64_int64s (float64 F, bool Rounded)
 {
     //Out of boundaries
-    if (F>=(int64s)0x7FFFFFFFFFFFFFFFLL)
-        return (int64s)0x7FFFFFFFFFFFFFFFLL;
-    if (F<=(int64s)0x8000000000000000LL)
-        return (int64s)0x8000000000000000LL;
+    if (F>=(float64)std::numeric_limits<int64s>::max())
+        return std::numeric_limits<int64s>::max();
+    if (F<=(float64)std::numeric_limits<int64s>::min())
+        return std::numeric_limits<int64s>::min();
 
     //Not rounded
     if (!Rounded)
diff --git a/Source/ZenLib/Ztring.cpp b/Source/ZenLib/Ztring.cpp
index 22a39bb..87760bb 100644
--- a/Source/ZenLib/Ztring.cpp
+++ b/Source/ZenLib/Ztring.cpp
@@ -1306,18 +1306,27 @@ Ztring& Ztring::Date_From_Seconds_1970 (const int32s Value)
 Ztring& Ztring::Date_From_Seconds_1970 (const int64s Value)
 {
     time_t Time=(time_t)Value;
-    #if defined(HAVE_GMTIME_R)
-    struct tm Gmt_Temp;
-    struct tm *Gmt=gmtime_r(&Time, &Gmt_Temp);
-    #elif defined(_MSC_VER)
-    struct tm Gmt_Temp;
-    errno_t gmtime_s_Result=gmtime_s(&Gmt_Temp , &Time);
-    struct tm* Gmt=gmtime_s_Result?NULL:&Gmt_Temp;
+    #if defined(_WIN32)
+        #if _CRT_USE_CONFORMING_ANNEX_K_TIME || (__STDC_WANT_LIB_EXT1__ && defined(__STDC_LIB_EXT1__))
+        // C11 standard version on MSVC or other compilers
+        tm Gmt_Temp;
+        tm* Gmt = gmtime_s(&Time, &Gmt_Temp);
+        #else
+        // MSVC and MinGW-w64 argument order and return value differs from C11 standard
+        tm Gmt_Temp;
+        errno_t gmtime_s_Result = gmtime_s(&Gmt_Temp, &Time);
+        tm* Gmt = gmtime_s_Result ? nullptr : &Gmt_Temp;
+        #endif
+    #elif defined(HAVE_GMTIME_R)
+        // POSIX or C23
+        tm Gmt_Temp;
+        tm *Gmt = gmtime_r(&Time, &Gmt_Temp);
     #else
-    #ifdef __GNUC__
-    #warning "This version of ZenLib is not thread safe"
-    #endif
-    struct tm *Gmt=gmtime(&Time);
+        // Fallback: not thread-safe, but prevents compile errors
+        #ifdef __GNUC__
+        #warning "This version of ZenLib is not thread safe"
+        #endif
+        tm *Gmt = gmtime(&Time);
     #endif
     if (!Gmt)
     {
@@ -1349,18 +1358,27 @@ Ztring& Ztring::Date_From_Seconds_1970 (const int64s Value)
 Ztring& Ztring::Date_From_Seconds_1970_Local (const int32u Value)
 {
     time_t Time=(time_t)Value;
-    #if defined(HAVE_LOCALTIME_R)
-    struct tm Gmt_Temp;
-    struct tm *Gmt=localtime_r(&Time, &Gmt_Temp);
-    #elif defined(_MSC_VER)
-    struct tm Gmt_Temp;
-    errno_t localtime_s_Result=localtime_s(&Gmt_Temp , &Time);
-    struct tm* Gmt=localtime_s_Result?NULL:&Gmt_Temp;
+    #if defined(_WIN32)
+        #if _CRT_USE_CONFORMING_ANNEX_K_TIME || (__STDC_WANT_LIB_EXT1__ && defined(__STDC_LIB_EXT1__))
+        // C11 standard version on MSVC or other compilers
+        tm Gmt_Temp;
+        tm* Gmt = localtime_s(&Time, &Gmt_Temp);
+        #else
+        // MSVC and MinGW-w64 argument order and return value differs from C11 standard
+        tm Gmt_Temp;
+        errno_t localtime_s_Result = localtime_s(&Gmt_Temp, &Time);
+        tm* Gmt = localtime_s_Result ? nullptr : &Gmt_Temp;
+        #endif
+    #elif defined(HAVE_GMTIME_R)
+        // POSIX or C23
+        tm Gmt_Temp;
+        tm *Gmt = localtime_r(&Time, &Gmt_Temp);
     #else
-    #ifdef __GNUC__
-    #warning "This version of ZenLib is not thread safe"
-    #endif
-    struct tm *Gmt=localtime(&Time);
+        // Fallback: not thread-safe, but prevents compile errors
+        #ifdef __GNUC__
+        #warning "This version of ZenLib is not thread safe"
+        #endif
+        tm *Gmt = localtime(&Time);
     #endif
     Ztring DateT;
     Ztring Date;