How is the time fed between hardclocks ?

[deco33000 at yandex.com]

Hello,

I am learning the freebsd way to keep the time up to date.

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

But, how do we know how far we are from the last hardclock()?
Are the updates to the "fake" timestamps occuring at a constant rate?

This is the last -and most important- point I don't get.

Thanks

-- 
Jog