diff --git a/kernel/clock.h b/kernel/clock.h
index f931244..f79abec 100644
--- a/kernel/clock.h
+++ b/kernel/clock.h
@@ -9,6 +9,10 @@ See the file LICENSE for details.
 
 #include "kernel/types.h"
 
+#ifdef KERNEL_CLOCK_EXT
+  #include "kernel_clock_ext.h"
+#endif
+
 typedef struct {
 	uint32_t seconds;
 	uint32_t millis;