Micro 820 RTC how to sync?

Hello Friends

We have 4 Micro 820 (2080-LC20-20QWB) and We are using the built in RTC, the problem is that it is not exact.

In one month We have a difference of 15 seconds or more between RTC PLCs.

Is it posible to sync the RTC in the 4 PLC? How could We do that?

Thanks in advance