How to calculate the number of days left?
You want to use "a" as your DateInterval format parameter instead of "d".

Also, you may need to use Twig's date function on the end date, which "converts an argument to a date to allow date comparison".

This should give you the correct number of days between now and the future date:

{{ date( inschrijven.datumEvenement|date ).diff( now ).format('%a') }}


or

{{ date( inschrijven.datumEvenement|date ).diff( now ).days }}