Wiley,
I would say about 3/4th of the PLC models now have "real-time" clocks. (If it has a clock, it is almost invariably a real-time clock. I have never seen one with an unreal-time clock). The clock will have some bits that are available to the programmer, such as a 1-second, 1-minute, 0.1 second, and so on. foir example, the ALlen-=Bradley SLC 500 brand has a 16-bit "free-running" clock at Status File location S:4. Any of the S:4 bits can be used by a programmer on a rung of his program.