I have an Airport Extreme serving up an AFP share. The share is mounted on both a Mac and a Linux box, the latter with afpfs-ng.
Everything seems to be OK, except that the two systems don't agree on file modification dates: the ones the Mac sees are 5 hours later than the ones the Linux box sees. If I touch a file on the Linux box, on the Mac it looks like it's been touched in the future. If I touch a file on the Mac, it can actually move its modification date backwards as far as Linux is concerned.
The 5 hours is precisely the timezone offset from GMT over here (I'm in Eastern time). I'm pretty sure the Linux box is set to "System clock uses UTC", but changing that doesn't affect this behavior.
This is a bit of a problem because I'm doing rsync backups of the data on this disk, and timestamps being off means rsync will sometimes overwrite things in the wrong direction.