How to get GMT time in milliseconds using boost Date_Time?

There's nothing built-in that I can see, but as usual, it's trivial to implement:

boost::posix_time::time_duration::tick_type milliseconds_since_epoch()
{
    using boost::gregorian::date;
    using boost::posix_time::ptime;
    using boost::posix_time::microsec_clock;

    static ptime const epoch(date(1970, 1, 1));
    return (microsec_clock::universal_time() - epoch).total_milliseconds();
}