How to find uptime on a host?

If I take a "nmap -P0 -n -sU -sS -p 80 -O www.freebsd.org" on my FreeBSD
v5.4 I get this output:

Starting nmap 3.81 ( http://www.insecure.org/nmap/ ) at 2005-09-10 02:40
CEST
Warning: OS detection will be MUCH less reliable because we did not find at
least 1 open and 1 closed TCP port
Interesting ports on 216.136.204.117:
PORT STATE SERVICE
80/tcp open http
80/udp open|filtered http
Device type: general purpose
Running: OpenBSD 3.X
OS details: OpenBSD 3.6 x86 with pf "scrub in all"
Uptime 134.821 days (since Thu Apr 28 06:58:19 2005)
Nmap finished: 1 IP address (1 host up) scanned in 17.816 seconds

With this I can easily grep the uptime. If I do the same with
www.microsoft.com, it doesn't give me this uptime:

Starting nmap 3.81 ( http://www.insecure.org/nmap/ ) at 2005-09-10 02:42
CEST
Warning: OS detection will be MUCH less reliable because we did not find at
least 1 open and 1 closed TCP port
Interesting ports on 207.46.18.30:
PORT STATE SERVICE
80/tcp open http
80/udp open|filtered http
Device type: general purpose
Running (JUST GUESSING) : IBM AIX 4.X|3.X (88%)
Aggressive OS guesses: IBM AIX 4.3.2.0-4.3.3.0 on an IBM RS/* (88%), IBM AIX
v3.2.5 - 4 (87%), IBM AIX 4.2-4.3.3 (87%), IBM AIX v4.2 (87%)
No exact OS matches for host (test conditions non-ideal).
Nmap finished: 1 IP address (1 host up) scanned in 55.904 seconds

I know that Netcraft is able to see this uptime but how do they do and how
do I get this uptime from any host?