Real Time Clock
What's wrong with using the RTC addressing in the status registers:
Year: S:18
Month: S:19
Day: S:20
Hour: S:21
Minute: S:22
Second: S:23
The old RTC module was designed for the PLC-2 (which had no internal clock). If memory serves, it wasn't Y2K complient. With the PLC-2 being "silver series'ed, I doubt you'll find one in any catalog. Probably be tough even on eBay.
BTW: RTC Module part number: 1771-DC