title: Is behaviour well-defined when `sleep_until()` specifies a time point in the past?
body: <p>The C++11 standard talks about what should happen if the system clock is adjusted such that the time point passed to <code>sleep_until()</code> is now in the past - but I can't see anywhere that addresses the case when the specified time point is already in the past.</p>
  
  <p>Have I simply overlooked something, or is it really not specified - even as UB or implementation-defined?</p>
  
  <p>A similar question arises if <code>sleep_for()</code> is invoked with a negative duration.</p>