If accurate time is important for a PLC that may be powered off long enough for the time to get lost, consider configuring an NTP server on the same network as the PLC, and then point the PLC to that NTP server.  An existing permanently connected / networked PC can be configured as an NTP server.

Or, networking hardware, like network switches, can be purchased which have an NTP server as an option.

Or, other options exist.  Search this support site for NTP and TIA Portal to see what Siemens suggests.

I usually have a PC always connected to a PLC, so I enable NTP server on the PC, and point the PLC's NTP configuration to the PC's IP.

If you have Sinamics S120 drives and can use the latest firmware, they can also benefit from NTP configuration.  That way, the diagnostics buffer entries have realistic time entries.

xsmart

Hello. I have a question, that is close to topic

How long CPU's clock is working, when CPU power is switched off? Can't find this info. Another words, how long CPU can remember and count current date and time with no external power??

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Split from S7-1200 power up after long time swtiched off.