Time Drifting on NW 5 Server

I’ve got a single-server net running NetWare 5.0 SP6a. Our server time seems to fall behind about 15 minutes every day. We are connected to a WAN via proxy, with limited port access, so NTP is unavailable. Any possible suggestions on how to fix this? This server was recently upgraded from NW 4.2, and the clock was solid before the upgrade.