-periodic watcher, ever, or make any event loop modificstions>. If you need
-to stop it, return 1e30 (or so, fudge fudge) and stop it afterwards.
+periodic watcher, ever, or make any event loop modifications>. If you need
+to stop it, return C<now + 1e30> (or so, fudge fudge) and stop it afterwards.
+
+Also, I<<this callback must always return a time that is later than the
+passed C<now> value >>. Not even C<now> itself will be ok.