Downtime recording for every 12 hours shift

I want to record into a PLC 500 production downtime with posibility to know from what time(HH:mm) and how(min) knowing that production needs to run 24/7. All this downtime needs to be stored into PLC and need to be reported at 12 hours shift(8:00 to 20:00) or (20:00 to 08:00).
Can anybody give me a sugestion how to do it?

Use a timer RTO to count the downtime seconds. After 60 sec trigger a counter for min, after 60 counts that equal a hour, trigger a hour counter... Reset all after 12 hrs?

Thank you very much. Do you know any possibility to record somewhere in the PLC when(HH:mm) is starting downtime?