Getting absolute time stamps on host

[aalbaali]

I'm trying to get the image/frame timestamps in the host clock. I understand that frame.getTimestamp() returns a relative time since the host was booted. However, I compared the output of frame.getTimestamp() with uptime (on linux)`, and the output didn't match my expectation (even after syncing the clocks):

How can I get the absolute timestamp on the host machine? I expect the solution to be something like

abs_timestamp = frame.getTimestamp() + abs_reference_timestamp

but I'm not sure how to get abs_reference_timestamp.