Get a datetime with nanoseconds topic

Hi, ALL,
I am woking on an application for digital forensic.
In this application I am getting this 2 pieces of information:

atime - long representing the time stamp
atime_nano - long representing the nanoseconds.

What I'd like to do is to have a python datetime object which will be a
representation of those 2 values.
I can get a datetime object out of atime timestamp, but I don't know how to
do it for atime_nano.

I did a little research. It looks like people on SO are saying that I will
not be able to get this kind of precision, but I'd be grateful if I can at
least get the best possible precision (millioseconds?) it would be great.

Thank you for any pointers.