diff --git a/src/bacnetmstp/device-client.c b/src/bacnetmstp/device-client.c
index cb25dd96..a6c0b976 100644
--- a/src/bacnetmstp/device-client.c
+++ b/src/bacnetmstp/device-client.c
@@ -55,6 +55,8 @@
 #include <stdbool.h>
 #include <stdint.h>
 #include <string.h>     /* for memmove */
+// __USE_XOPEN needed for timezone
+#define __USE_XOPEN
 #include <time.h>       /* for timezone, localtime */
 
 #define BACDL_MSTP 1