Is behaviour well-defined when `sleep_until()` specifies a time point in the past?

The C++11 standard talks about what should happen if the system clock is adjusted such that the time point passed to sleep_until() 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.

Have I simply overlooked something, or is it really not specified - even as UB or implementation-defined?