So if we can't prove accurate records were kept for say, the past two months i.e. the system time cannot be verified against an external source (atomic clock, GMT etc), then the evidence is considered inaccurate and admissible?
Maybe not inadmissible but certainly weakened. You are guaranteed he will bring this up at tribunal. It's up to you to prove your case.
I suppose if you can show that your system does not lose/gain time from now onwards, it would be a start and would add value to your old figures. However, there is no proof that even if the system keeps good time, the old recordings were made when the computer was set to the wrong time.
Equally, once you have now started keeping accuracy records, if he keeps being late then you don;t need the old evidence.
I am starting to guess that you want to either discipline the guy or sack him and it's starting to look like he might get away with his previous poor timekeeping?
If so, it will only be a matter of time before he messes up again.
ANother thing you may want to look into is CCTV specifically mentioned in their contract of employment as being used for timekeeping purposes?
If the CCTV is a recent thing, did you inform the emloyees in writing that it was being installed and what purposes the footage was going to be used for? Did you give the required notice period prior to the CCTV being installed?
If the answer is no to any of the above then you can't legally use the CCTV footage against the employee and all your timekeeping records are useless.
We fell into this trap when we wanted to use CCTV to discipline and employee but since we only installed it for security and health and safety, after we took legal advice, we found we couldn't use the footage. That is now all corrected and we can now.