diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index dc724eee2..1b13deb3e 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -28,7 +28,9 @@
  * Ensure gmtime_r is available even with -std=c99; must be included before
  * config.h, which pulls in glibc's features.h. Harmless on other platforms.
  */
+#if !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 200112L
+#endif
 
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "config.h"
diff --git a/library/platform_util.c b/library/platform_util.c
index 9bcf15e6a..6a5feb321 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -24,7 +24,9 @@
  * Ensure gmtime_r is available even with -std=c99; must be included before
  * config.h, which pulls in glibc's features.h. Harmless on other platforms.
  */
+#if !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 200112L
+#endif
 
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
diff --git a/library/threading.c b/library/threading.c
index 7231f2f9b..c1834bace 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -23,7 +23,9 @@
  * Ensure gmtime_r is available even with -std=c99; must be included before
  * config.h, which pulls in glibc's features.h. Harmless on other platforms.
  */
+#if !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 200112L
+#endif
 
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"