diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index e613be9c2..71538c07a 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -24,6 +24,12 @@
 #ifndef MBEDTLS_THREADING_H
 #define MBEDTLS_THREADING_H
 
+/*
+ * 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.
+ */
+#define _POSIX_C_SOURCE 200112L
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "config.h"
 #else