diff --git a/Foundation/src/Clock.cpp b/Foundation/src/Clock.cpp
index 6000ef38b..ac95bfc80 100644
--- a/Foundation/src/Clock.cpp
+++ b/Foundation/src/Clock.cpp
@@ -22,6 +22,7 @@
 #include <mach/clock.h>
 #elif defined(POCO_OS_FAMILY_UNIX)
 #include <time.h>
+#include <unistd.h>
 #elif defined(POCO_VXWORKS)
 #include <timers.h>
 #elif defined(POCO_OS_FAMILY_WINDOWS)