How to get nanoseconds from boost::chrono::hight_resolution_clock::time_point?

I am new to boost and chrono. I am writing a logger that logs the timestamps of API calls, entry and exit. I tried using boost::xtime first, but it wasn't giving the high resolution value I needed. Hence was thinking about using Chrono. I declared a boost::chrono::hight_resolution_clock::time_stamp x; variable for getting the timestamp and assigned it to boost::chrono::hight_resolution_clock::now ();. Now, I need to get the nanoseconds from this variable and put it in my log file (thats the requirement). So I cast it boost::chrono::duration_cast (x). But it just wouldn't let me do that. It needs 2 parameters apparently, and I only have one. Is there a way to get around this?. Is it possible to create another time_stamp variable and assign zero to it and use that variable?. I tried assigning zero, but its not working. Kindly help me out.

Thanks, Sam