OWA and calendars on a Mac

Running Exchange 2003 on a W2K server. When accessing calendars, the Macs cannot see or set exact times. They can only pick from a list of half hour increments. For instance, an 8:55 meeting must be set for 9:00. This does not happen on PC’s. Tried different browsers, and they all do the same thing. This is a problem because I am trying to set up public calendars.