diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index 2e61b2e52..11f6341d9 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -112,14 +112,18 @@ extern mbedtls_threading_mutex_t mbedtls_threading_readdir_mutex;
     defined(__unix) || defined(__unix__) || (defined(__APPLE__) && \
     defined(__MACH__)))
 #include <unistd.h>
-#if !defined(_POSIX_VERSION) || 200112L > _POSIX_THREAD_SAFE_FUNCTIONS
+#if !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+       ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+         _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) )
 /*
  * The preprocessor conditions above are the same as in platform_util.c and
  * threading.c. Remember to update the code there when changing the conditions
  * here.
  */
 extern mbedtls_threading_mutex_t mbedtls_threading_gmtime_mutex;
-#endif /* !_POSIX_VERSION || 200112L > _POSIX_THREAD_SAFE_FUNCTIONS */
+#endif /* !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+             ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+                _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) ) */
 #endif /* !_WIN32 && (unix || __unix || __unix__ ||
         * (__APPLE__ && __MACH__)) */
 #endif /* MBEDTLS_HAVE_TIME_DATE && !MBEDTLS_PLATFORM_GMTIME_R_ALT */
diff --git a/library/platform_util.c b/library/platform_util.c
index ddb56ed0b..7e82293e1 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -81,7 +81,10 @@ void mbedtls_platform_zeroize( void *buf, size_t len )
     defined(__unix) || defined(__unix__) || (defined(__APPLE__) && \
     defined(__MACH__)))
 #include <unistd.h>
-#if !defined(_POSIX_VERSION) || _POSIX_C_SOURCE > _POSIX_THREAD_SAFE_FUNCTIONS
+
+#if !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+       ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+         _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) )
 /*
  * This is a convenience shorthand macro to avoid checking the long
  * preprocessor conditions above. Ideally, we could expose this macro in
@@ -90,7 +93,9 @@ void mbedtls_platform_zeroize( void *buf, size_t len )
  * we keep it private by only defining it in this file
  */
 #define PLATFORM_UTIL_USE_GMTIME
-#endif /* !_POSIX_VERSION || _POSIX_C_SOURCE > _POSIX_THREAD_SAFE_FUNCTIONS */
+#endif /* !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+             ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+                _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) ) */
 #endif /* !_WIN32 && (unix || __unix || __unix__ ||
         * (__APPLE__ && __MACH__)) */
 
diff --git a/library/threading.c b/library/threading.c
index f37049b8d..60dfd02af 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -42,7 +42,9 @@
     defined(__unix) || defined(__unix__) || (defined(__APPLE__) && \
     defined(__MACH__)))
 #include <unistd.h>
-#if !defined(_POSIX_VERSION) || 200112L > _POSIX_THREAD_SAFE_FUNCTIONS
+#if !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+       ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+         _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) )
 /*
  * This is a convenience shorthand macro to avoid checking the long
  * preprocessor conditions above. Ideally, we could expose this macro in
@@ -51,7 +53,9 @@
  * we keep it private by only defining it in this file
  */
 #define THREADING_USE_GMTIME
-#endif /* !_POSIX_VERSION || 200112L > _POSIX_THREAD_SAFE_FUNCTIONS */
+#endif /* !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+             ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+                _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) ) */
 #endif /* !_WIN32 && (unix || __unix || __unix__ ||
         * (__APPLE__ && __MACH__)) */
 #endif /* MBEDTLS_HAVE_TIME_DATE && !MBEDTLS_PLATFORM_GMTIME_R_ALT */