The safe-t-cam system calculates average speed between key points by recording trucks licence plates as they go past. However the system made use of the local time (rather then some sort of epoc time) in its time calculations so gave results showing trucks taking 3 hours to travel a distance that it would normally take 4 hours to go.
Reminds me of Amtrak’s handling of time changes - the train either waits at a station until leave time (if it was less than an hour late) or it just becomes an hour late depending on which time change it was.