This train counts as one travel day as you enter before midnight and don't change trains after midnight CET.

Counting travel days are rather easy. Every day 00.00-23.59 CET when you board a train (or other means of transportation where the pass gives 100 % discount on the ticket) is a travel day. It is the scheduled time of the trains departure that counts.

The recommendation is not to activate the pass until you are absolutely sure that you will start travelling on the day you have planned. This, as the pass only can be deactivated until 23.59 CET the day before the activation date.

You can test to activate the pass with a date in the future just to see that it works and then immediately deactivate the pass.

If you have a flexi pass, like 15 days in 2 months, the recommendation is also that you don't connect the journey to your pass until just before boarding the train. If you connect a journey to the pass in advance and then your travel plans change so that you can't travel on that day, you can end up loosing a travel day.

@AnnaB Thank you, I actually do have a flexi pass. So you are saying that everytime my travel day is over I must disconnect my journey from the pass? Will this not end my pass? 

Thanks again for the quick response, have a good day!

No, just test activation once (with a start date in the future) and then undo it until you are sure what your start date will be. Once you started using the pass, you can't cancel it anymore. For any new travel day, only add a journey to your pass shortly before boarding a train. If a new travel day needs to be activated, the app will warn you.

Very helpful, thank you for responding always so quickly.