for uptimes greater than 365 days, rup shows the wrong number of days - apparently (days modulo 365). However, "rup -t" sorts correctly, so this is probably just a display bug. (The rup in RH6.0 has the problem too)
This issue has been forwarded to a developer for further action.
The program rup in netkit-rusers-0.15 reports days using the tm_yday field returned by gmtime(3) using as input the delta of now - boottime of the responding host. I dunno how to improve this algorithm without opening up a lot of snarly issues involving local/remote time synchronization and the gain is not worth the effort.