Timekeeping between two hardclocks

[deco33000 at yandex.com]

Hello,

I am learning the freebsd way to keep the time up to date.
I already read http://phk.freebsd.dk/pubs/timecounter.pdf

I am mostly in sys/kern_tc.c 

We avoid to call hardware hardclock() every time we need a timestamp by applying some math (to improve performance). I get the idea.

But, how do we know how far we are from the last hardclock(), at any given moment?
Are the updates to the "fake" timestamps occuring at a constant rate? Is there a feed somewhere I missed? I don't see where in that case.

This is the last -and most important- point I don't get on the timer/clock subsystem.

I seriously need your help/pointers I guess.

Thanks

-- 
Jog