diff --git a/src/unix/linux.c b/src/unix/linux.c
index 44c02837..671ed23a 100644
--- a/src/unix/linux.c
+++ b/src/unix/linux.c
@@ -139,6 +139,11 @@
 # include <netpacket/packet.h>
 #endif /* HAVE_IFADDRS_H */
 
+/* Available from 2.6.39 onwards. */
+#ifndef CLOCK_BOOTTIME
+# define CLOCK_BOOTTIME 7
+#endif
+
 enum {
   UV__IORING_SETUP_SQPOLL = 2u,
 };