diff --git a/basis/calendar/threads/threads.factor b/basis/calendar/threads/threads.factor
index efdbb6923d..0501d4180a 100644
--- a/basis/calendar/threads/threads.factor
+++ b/basis/calendar/threads/threads.factor
@@ -1,7 +1,6 @@
 ! Copyright (C) 2011 Slava Pestov.
 ! See http://factorcode.org/license.txt for BSD license.
-USING: calendar math system threads ;
+USING: calendar threads ;
 IN: calendar.threads
 
-M: duration sleep
-    duration>nanoseconds >integer nano-count + sleep-until ;
+M: duration sleep duration>nanoseconds sleep ;