How do I convert boost::posix_time::ptime to time_t?

Since @icecrime’s method converts twice (ptime uses linear representation internally), I’ve decided to use direct computation instead. Here it is:

time_t to_time_t(boost::posix_time::ptime t)
{
    using namespace boost::posix_time;
    ptime epoch(boost::gregorian::date(1970,1,1));
    time_duration::sec_type x = (t - epoch).total_seconds();

    // ... check overflow here ...

    return time_t(x);
}

EDIT: Thanks @jaaw for bringing this to my attention. Since boost 1.58 this function is included in date_time/posix_time/conversion.hpp, std::time_t to_time_t(ptime pt).