In Germany, Austria, Switzerland, time-limited free parking is regulated by the same so-called parking disks.
You have to put your arrival time on the disk to the next half hour at the parking lot. It means you can gain up to 30 mins if you arrive seconds after the full hour or half hour.
OK, the 'official' parking disks are blue, the same color as the parking signs have. They have to have a certain size, time adjusted parking disks are allowed but you have to stop the clock mechanism when you leave your car.
The lady who put her not-official, pink parking disk at a parking lot in the city of Herten, Ruhr Valley, North Rhine-Westphalia got a 5 euro ticket due to this rule-violation.
At rental car companies, you are well advised to ask for such a device - normally already there in the glove box of the car else you get it at gas stations or "Walmart-like" shopping centers.
(adapted from a Bild-online article about the 10 craziest German traffic regulations)