diff --git a/compat.h b/compat.h
index b9ee7ba1..622006e1 100644
--- a/compat.h
+++ b/compat.h
@@ -152,6 +152,18 @@ typedef uint64_t u_int64_t;
 	} while (0)
 #endif
 
+#ifndef timersub
+#define timersub(tvp, uvp, vvp)                                         \
+	do {                                                            \
+		(vvp)->tv_sec = (tvp)->tv_sec - (uvp)->tv_sec;          \
+		(vvp)->tv_usec = (tvp)->tv_usec - (uvp)->tv_usec;       \
+		if ((vvp)->tv_usec < 0) {                               \
+			(vvp)->tv_sec--;                                \
+			(vvp)->tv_usec += 1000000;                      \
+		}                                                       \
+	} while (0)
+#endif
+
 #ifndef TTY_NAME_MAX
 #define TTY_NAME_MAX 32
 #endif