A parking disc ("parkeringsskive", "P-skive") is required in many public and private parking spaces in Denmark.
Driving your own car as a guest in Denmark you may not have a parking disc at hand in which case you may fill out a paper in clear writing with your time of arrival (e.g. "11:45") and place it in your windshield easily legible from the outside.
You may also purchase a parking disc on a gas station or in a car accessory store.
Time restrictions as signed should of course be obeyed at all times.