The problem is with one of the fields being blank (or zero, whichever you guys use). Apparently time stamps are in seconds from the date 12/31/1969 or 1/1/1970 (the former seems to be our case). If it's zero, it's zero seconds since that date, thereby giving us said date. Apparently, a blank field defaults to zero. If Groupwise can be configured server-side to put a correct date in this field, that should solve the problem.
Did a little digging around on the internet and it appears that there's no client-side fix for this right now, except perhaps to get a new client.