Get the timer's expiry time as an absolute time.
time_point expires_at() const;
This function may be used to obtain the timer's current expiry time.
Whether the timer has expired or not does not affect this value.
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)