I recently installed WRAP 1D1 board with m0n0wall v1.1, and noticed that 
the system uptime was incorrect when retreived  via SNMP. For instance, 
this system has really be up about 2 days. The Status-->System page in 
the webGui correctly shows:

Name  	 fw.example.com
Version 	1.1
built on Sun Aug 22 11:41:15 CEST 2004
Platform 	wrap
Uptime 	2 days, 08:29

Yet, an SNMPGET (which is what MRTG uses) incorrectly shows:

$ snmpget -c public -v 2c 192.168.0.254 sysUpTime.0
SNMPv2-MIB::sysUpTime.0 = Timeticks: (357526304) 41 days, 9:07:43.04

So, my MRTG pages show that the system has been up 41 days, when in 
reality it has been up for 2 days 8 hours.

I checked the list archives and the FAQ and found nothing specific 
relating to problems with this OID.

A quick check using snmpwalk shows that these two OIDs also do not seem 
possible:
ifLastChange.1 = Timeticks: (2055159851) 237 days, 20:46:38.51
ifLastChange.2 = Timeticks: (0) 0:00:00.00