Like others have mentioned the best practice would be to validate Before Committing instead of putting a validation rule on the variable.
Assuming you have a page where you let a user fill out your inschrijvingsformulier.
1.) Create a microflow that looks like this:
2.) Add new event handler with Before Commit. Select this microflow. Now you prevent the data from entering the data base.
Optional but recommended
1.) Either put an onchange/on leave event on the input field where you let the user enter this date. And perform the same check there and add validation feedback after the decision.
2.) On the save button where you save the form, put the logic before you commit the object.
The values are either specified fixed values or values of other attributes of the same object.
As for the setting the actual rule I recommend using either
Hope these help!