This sign is from a local authority car park. "Blue Badge holders Max Stay 4 hours...clocks must be displayed correctly".
For the blue badge parking system, parking is either unlimited stay or time limited stay. Where it's unlimited stay, just put the blue badge in the windscreen. Where local parking restrictions impose a time limit on blue badge parking, there's a cardboard clock that needs to be used and is issued with the blue badge. In those cases simply set the time on the cardboard clock for the time that you park, and that goes in the windscreen with the blue badge.
So in this situation, there's a time restriction and clocks need to be used. Simple enough. So it could say "4 hour parking limit, please use your clock". But the instruction here is more than to simply use the clocks. There's a chastising undertone here, with the instruction to use the clock correctly..."clocks must be displayed correctly". Perhaps it could also to be explicit that everything else needs to be done correctly, display the blue badge correctly, and even "park correctly". So correctly here simply means setting the clock to the arrival time. Job done.
That chastising sense of using "correctly" creates it's own problems. I fact the sign has been modified to hide some additional text. So it originally read "In this section of the car park...." before "clocks must be displayed correctly". That would imply that in the other section of the car park the clocks do not need to be displayed correctly. That's probably why the text needed to be removed in the first place. And that's all because of the apparent need to use the word "correctly", when correct use is natural, implied and easy. Otherwise it would have sufficed with "In the this section of the car park clocks must be displayed.
