void save(Archive& ar, boost::chrono::time_point<clock> const& tp, unsigned)
{
using namespace boost::chrono;
milliseconds::rep millis = duration_cast<milliseconds>(tp.time_since_epoch()).count();
ar & millis;
}
boost::posix_time::ptime
to_ptime(boost::chrono::time_point<Clock, Duration> const& from)
{
typedef boost::chrono::nanoseconds duration_type;
typedef duration_type::rep rep_type;
rep_type d = boost::chrono::duration_cast<duration_type>(
from.time_since_epoch()).count();
rep_type sec = d / 1000000000;
rep_type nsec = d % 1000000000;
return boost::posix_time::from_time_t(0) +
boost::posix_time::seconds(static_cast<long>(sec)) +
#ifdef BOOST_DATE_TIME_HAS_NANOSECONDS
boost::posix_time::nanoseconds(nsec);
#else
boost::posix_time::microseconds((nsec+500)/1000);
#endif
}