accuracy of access.log fields

Hello.

I'm a post-graduate student investigating web traffic.
I need to know, which moment of time does field "duration"
means.
In documentation I've found next:
"duration

The elapsed time considers how many milliseconds the transaction busied the cache. It differs in interpretation between TCP and UDP:
1. For HTTP/1.0, this is basically the time between accept() and close().
2. For persistent connections, this ought to be the time between scheduling the reply and finishing sending it.
3. For ICP, this is the time between scheduling a reply and actually
sending it."

Does it capture the time interval between the moment of client's
request arrival and moment of sending reply? (case 1 & 2)? Or may be
Squid starts "counting" the time then it starts sending the reply, and
the moment of client's request arrival is few milliseconds earlier?

Please, answer.
I need this information for comparison the data from access.log and
data from Cisco's NetFlow.

Thanks.

-- 
Alexander Volkov
                         mailto:volk@ipk.karelia.ru