there are issues with acars but the messages sent are time stamped by the system reporting the failure. As all systems are on the same time bus you can get an accurate picture.
What you can't account for is the sampling rate of the system but this generally applies to normal conditions i.e. some air conditioning sensors are monitored every 3 minutes. However a significant fault or warning is time stamped in real time.
That doesn't prevent the scenario however that there were other messages relating to earlier incidents that didn't make it to the transmitter for whatever reason