The twenty-four hour line, counted in Amsterdam time
The line falls one full day before the booked hour, in Amsterdam time; before it a cancellation returns the whole deposit, after it the notice scale applies.
Several rules in the booking terms turn on a single moment: twenty-four hours before the booked time. For a traveller planning from home, it is the one date on the calendar worth writing down alongside the booking itself.
Where it falls
Count back one full day from the booked hour, in Amsterdam time. A booking at nine on a Saturday evening has its line at nine on the Friday evening. A booking at half past midnight going into Sunday has its line at half past midnight going into Saturday. The line is fixed to the clock in Amsterdam, not to the clock where you happen to be, so a guest six hours behind will see it pass in his own afternoon.
What changes when it passes
Before the line, a cancellation returns the whole deposit and a first change of date carries it across in full. After the line, a cancellation sent before she sets out keeps half the deposit, and a move asked for is read as a cancellation at that moment. The line does not affect anything else: the figure, the hour and the woman you named all stay as agreed.
Why it tends to fall on a travel day
For a booking on the first or second evening of a trip, the line usually falls during the journey: in a departure lounge, on a train or somewhere over the sea. That makes it easy to cross without noticing. A traveller with any doubt about whether the evening will happen should decide before leaving home, while the whole deposit can still come back.
Clock changes
On the weekends in late March and late October when Amsterdam moves its clocks, the line is still one full day of real time before the booking, but it may not land at the hour on the clock you expect. Work it out on the actual date and leave a margin. The journal note on the clocks changing explains why, and the booking terms page sets out what applies on each side of the line.