Time Sync of Netware 4.x Server

I have a NW 4.11 server that keeps gaining twenty minutes or so a month. Is there a way to find out whats causing the acceleration of time?. I was thinking of a NLM, but the server has no Internet Access and don’t want to expose to the Internet. Theclients do have Internet Access through a router.I could sync that way, but I want the best solution. I know this is a little problem, but as all of you know, its quite annoying so I am awarding 500 points to rid this once and for all.