They are of course illegal.
For that matter, even if it only moves in steps you could still be unlucky in several ways. For example if the parking inspector is looking at it at the same time as it makes a big leap forward in time.
If the area is subject to multiple inspections every day then the parking inspector might also notice that the same car is sitting in the same spot between multiple inspection rounds over many hours. No amount of technologically advanced timers will help the cheater then.
Perhaps someone could also invent car paint that changes colour every hour :p and then hope the parking inspector and everyone else are not looking the moment the colour changes.
That, also exists:
https://www.theverge.com/2023/1/5/23540358/bmw-color-changin...
This is common in several countries in Europe. Typically a white dial / black digits rotating inside a square blue disk [0].
In Belgium the streets where that metering system is used (but the city also uses other metering systems: it's on a street by street basis) have a little picture of that disk printed on the street at regular intervals. It's very common to have some streets where parking is totally free, others that use this plastic clock disk and yet others that use a more common metering system.
It's a thing to set your timer on your phone to 1h55 minutes, knowing you have 5 minutes left to go advance the disk by two hours (yup, cheating).
Another common one is that typically at night parking is allowed, so you park on monday evening and directly set the clock to 8am. So you're good until 10am.
[0] https://www.google.com/search?q=disque+de+stationnement