That's the rule. A travel day runs from 0:00 until 23:59 and you don't need an extra travel day as long as you don't change after midnight. If you change after midnight, then you need another travel day or a normal ticket.

@Jerome Scherrer  you need another travelday (how ever the system has still a disfuntion about using a Nighttrain in your Country of residence) 

currently you had to add the Nighttrain just till Feldkirch (last Austrian stop) 
and add it once more then for feldkirch to buchs (with the next day) as the app currently attach the Inbound journey to the date of the travelday not when you really ross the border :/ 
You needed anyway 2traveldays for Graz - Buchs - Uzwill as Buchs -Uzwill is on a new day ;) 

This RULE=Reglung/Bestimmung was changed around 3 yrs ago, before it was the ´after 19.00´ rule: if taking a nighttrain leaving after that time, then you could travel as on the next day and thus also make further changes. But at that time overnighttrains were nearly all died out.

NOW in the current ruling you can make a nice daytrip on the day before the nighttrain leaves.

There are 1000s of possible longer journeys In EUR that cannot be done in 1 travelday.  We have seen here utter naive questions about really impossible trips.

But any Swiss so proud of its national trainsystem has a Halbtax or Netzabo, oder? Und Postbus oder ZüriTram is also not included to get home.