Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Set the expiry time for the timer relative to now.

std::size_t expires_from_now(
    implementation_type & impl,
    const duration_type & expiry_time,
    boost::system::error_code & ec);