how to find the number nanoseconds elapsed since 01/JAN/1970 00:00:00 (UTC)?

I trying to understand how i can find the number nanoseconds elapsed since 01/JAN/1970 00:00:00.

I don't find any example on the internet of how to do it ( or explain how this can be calculate with no reference to code ) I'm work on C++11 on windows.