Hello Krist
Good question!
It is not defined because the contract guarantees the transportation on the same day, yet not the punctual arrival at the destination. For this, the delay compensation exists. In practice, the tickets are also accepted on the previous connection if there was a good reason for it; it is a case-by-case decision.
However, it is quite a difficult topic: a general rule that also allows earlier connections could tempt customers to circumvent the train binding in general, claiming that an interruption during the journey is always possible and therefore travelling earlier is to be on the safe side.
But, it is a point that I wanted to check with the tariff experts from for a long time, and I will do that today. I will let you know the outcome.
Kind regards,
Roland