The information on the page is outdated.
The current version of the non-train-bound ticket is valid for 14 days ("within the event timeframe", but see below) from the day of purchase for one trip there and one trip back, each to be completed within 2 subsequent days (i.e. if you start the trip to Congress on the 26th you have to complete it by the 27th). Also, I believe the "event time frame" may be less restrictive than stated on the current version of the page.
In doubt, ask the hotline, it's less painful to book with them than to look at the server errors in the online system.