Most of the PLCs these days include a Real Time Clock that's lithium coin cell powered. For timing applications, this is essential; otherwise run/stop/prgm or power cycles will disrupt critical daily timing cycles and require a complex re-sync or re-resetting of real time. The Ladder IDE can include special register to access the real time seconds/minutes/hours/day/month/year. An option to "sync" with the computer clockisalso handy.
If you want to get really fancy, consider to add an Alarm setting to the clock, and theability to "sleep" the PLC until alarm wake-up, or a change on input(s ) or even communications line. This would be awesome for battery powered applications!