diff --git a/client/ui.h b/client/ui.h
index ca235c11f..f08c704bb 100644
--- a/client/ui.h
+++ b/client/ui.h
@@ -11,6 +11,13 @@
 #ifndef UI_H__
 #define UI_H__
 
+/* Ensure gmtime_r, strtok_r is available even with -std=c99; must be included before
+ */
+#if !defined(_WIN32)
+#define _POSIX_C_SOURCE 200112L
+#endif
+
+
 #define _USE_MATH_DEFINES
 #include <stdarg.h>
 #include <stdlib.h>