You can use boost::posix_time::time_duration to get the time range. E.g like this
boost::posix_time::time_duration diff = tick - now;
diff.total_milliseconds();
And to get a higher resolution you can change the clock you are using. For example to the boost::posix_time::microsec_clock, though this can be OS dependent. On Windows, for example, boost::posix_time::microsecond_clock has milisecond resolution, not microsecond.
An example which is a little dependent on the hardware.
int main(int argc, char* argv[])
{
boost::posix_time::ptime t1 = boost::posix_time::second_clock::local_time();
boost::this_thread::sleep(boost::posix_time::millisec(500));
boost::posix_time::ptime t2 = boost::posix_time::second_clock::local_time();
boost::posix_time::time_duration diff = t2 - t1;
std::cout << diff.total_milliseconds() << std::endl;
boost::posix_time::ptime mst1 = boost::posix_time::microsec_clock::local_time();
boost::this_thread::sleep(boost::posix_time::millisec(500));
boost::posix_time::ptime mst2 = boost::posix_time::microsec_clock::local_time();
boost::posix_time::time_duration msdiff = mst2 - mst1;
std::cout << msdiff.total_milliseconds() << std::endl;
return 0;
}
On my win7 machine. The first out is either 0 or 1000. Second resolution. The second one is nearly always 500, because of the higher resolution of the clock. I hope that help a little.