Get total seconds since 1970-01-01, millisecond precision.

current_time.cpp

#include <boost/date_time/posix_time/posix_time.hpp>

double current_time()
{
    using namespace boost::gregorian; 
    using namespace boost::posix_time;
    
    const boost::posix_time::ptime now = boost::posix_time::microsec_clock::local_time();
    ptime time_t_epoch(date(1970,1,1));
    time_duration diff = now - time_t_epoch;
    return diff.total_milliseconds() / 1000.0;
}