 These included systems that ran the CP/M operating system, as well as early models of the Apple II, the BBC Micro, and the Commodore PET, among others.Add-on peripheral boards that included real-time clock chips with on-board battery back-up were available for the IBM PC and XT, but the IBM AT was the first widely available PC that came equipped with date/time hardware built into the motherboard.Prior to the widespread availability of computer networks, most personal computer systems that did track system time did so only with respect to local time and did not make allowances for different time zones.