diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index 309727f..ed355e6 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -96,6 +96,7 @@
  * Global mutexes
  */
 extern mbedtls_threading_mutex_t mbedtls_threading_readdir_mutex;
+extern mbedtls_threading_mutex_t mbedtls_threading_gmtime_mutex;
 
 #ifdef __cplusplus
 }