I have been informed that the ACARS reporting system automatically reports the time that the first door is opened and this forms part of the flight log.
If the case goes to MCOL, claimants can ask the airline for a copy of the flight log as part of the document/information disclosure process. I have never seen an airline volunteer this information when asked, other than via the court.
https://en.wikipedia.org/wiki/ACARS
Anyone in the know care to comment!