Q:

boost::posix_time::microsec_clock to double seconds with microsecond resolution

I tried to find the answer all over the internet.  I need a timestamp in seconds with microsecond resolution. 
boost::posix_time::ptime now = boost::posix_time::microsec_clock::local_time();
// not really getting any further here
double now_seconds = 0; // a value like 12345.123511, time since epoch in seconds with usec precision

UPDATE:
It is sufficient to use the beginning of the current day as the epoch -- i.e. a 24h timestamp. 

A:

N.B. This answer provides a generic method that allows an arbitrary epoch, since it was written before the update.  fonZ's answer is a good simplification when a timestamp relative to beginning of a current day is needed. 

I'm not aware of an existing function in the library that would do exactly what you ask for, but with the help of documentation it is simple to come up your own in a few lines. 
Subtract your ptime from a ptime representing the epoch, to get a time_duration representing the amount of time elapsed since epoch.  The time_duration class provides total_microseconds().  Scale the result appropriately to get seconds. 
Code Sample
#include <boost/date_time/local_time/local_time.hpp>
#include <boost/date_time/posix_time/ptime.hpp>
#include <boost/format.hpp>
#include <iostream>

double seconds_from_epoch(boost::posix_time::ptime const& t)
{
    boost::posix_time::ptime const EPOCH(boost::gregorian::date(1970,1,1));
    boost::posix_time::time_duration delta(t - EPOCH);
    return (delta.total_microseconds() / 1000000.0);
}

int main()
{
    boost::posix_time::ptime now(boost::posix_time::microsec_clock::local_time());
    std::cout << boost::format("%0.6f\n") % seconds_from_epoch(now);
    return 0;   
}

Sample on Coliru
Console output:
1497218065.918929

