diff --git a/library/x509.c b/library/x509.c
index c17697b..52b5b64 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -898,7 +898,7 @@
     int ret = 0;
 
     tt = mbedtls_time( NULL );
-    lt = mbedtls_platform_gmtime( &tt, &tm_buf );
+    lt = mbedtls_platform_gmtime_r( &tt, &tm_buf );
 
     if( lt == NULL )
         ret = -1;