The ReactorTimer only allows one outstanding Future at a time, and will cancel the outstanding Future if waitUntil is called again.
 
This comment on waitUntil makes this fairly clear:

 /* 
  * Returns a future that will be filled with Status::OK after the deadline has passed. 
  * 
  * Calling this implicitly calls cancel(). 
*/ 
virtual Future<void> waitUntil(Date_t deadline, const BatonHandle& baton = nullptr) = 0;

but then the comment on the destructor mentions a plural Futures:

/* 
 * The destructor calls cancel() to ensure outstanding Futures are filled.   */ 
virtual ~ReactorTimer() = default;

It would be nice to make it very explicit that there can only be one outstanding Future at a time on the ReactorTimer, and remove the reference to plural Futures from the destructor docstring.