Only in case you haven't downloaded or opened the ticket yet, you can contact customer service for a refund. This can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook.

It is not possible to change the information on an already downloaded eticket, or to have it cancelled, since NS cannot invalidate etickets. You could have printed it already and still use it to travel. You could contact customer service in this case as well to see if they're willing to make an exception for you, but I wouldn't get my hopes up if I were you.

I find this extremely foolish. How can we know that we have booked a ticket for a wrong date without opening or downloading a ticket? I am against this policy.

Quite simple: by checking the date before you continue to payment of your ticket…

It's anyhow more convenient to check in/out with your bankcard instead of buying an eticket.

When purchasing the ticket from the NS app, which I'm sure a lot of people do versus on their computers, you don't even get a confirmation of the date of the ticket on the page right before purchase or payment. 

For people who have never travelled with NS, or don't usually buy tickets, it's not always obvious that the tickets are date specific. Seeing that the option to change the ticket date is on the scheduling app, there isn't a clear link between that and the final purchase ticket.

In the app, I see the date (and other details) on the page where you select the payment method and where you have to tap on the Pay button. Don't you see it there?