C-more has a built-in real-time clock and calendar with battery back-up, but there's much more:

C-more can maintain the clock/calendar internally and periodically write tags to a PLC to keep it synchronized. If connected to a network. C-more can also synchronize (Coming Soon) it's own clock with a time server on a specified schedule.

C-more can also allow a PLC to maintain the clock and calendar, and C-more will use specified PLC tags instead of it's own internal real-time clock.