diff --git a/basis/calendar/calendar.factor b/basis/calendar/calendar.factor
index 4e6b35161f..85a1c72d11 100644
--- a/basis/calendar/calendar.factor
+++ b/basis/calendar/calendar.factor
@@ -2,7 +2,7 @@
 ! See http://factorcode.org/license.txt for BSD license.
 USING: accessors arrays classes.tuple combinators
 combinators.short-circuit kernel locals math math.functions
-math.order sequences summary system threads vocabs.loader ;
+math.order sequences summary system vocabs.loader ;
 IN: calendar
 
 HOOK: gmt-offset os ( -- hours minutes seconds )
@@ -540,10 +540,9 @@ M: integer end-of-year 12 31 <date> ;
 : unix-time>timestamp ( seconds -- timestamp )
     seconds unix-1970 time+ ;
 
-M: duration sleep
-    duration>nanoseconds >integer nano-count + sleep-until ;
-
 {
     { [ os unix? ] [ "calendar.unix" ] }
     { [ os windows? ] [ "calendar.windows" ] }
 } cond require
+
+{ "threads" "calendar" } "calendar.threads" require-when
diff --git a/basis/calendar/threads/threads.factor b/basis/calendar/threads/threads.factor
new file mode 100644
index 0000000000..efdbb6923d
--- /dev/null
+++ b/basis/calendar/threads/threads.factor
@@ -0,0 +1,7 @@
+! Copyright (C) 2011 Slava Pestov.
+! See http://factorcode.org/license.txt for BSD license.
+USING: calendar math system threads ;
+IN: calendar.threads
+
+M: duration sleep
+    duration>nanoseconds >integer nano-count + sleep-until ;