I already, some times ago, did mark in my calendar, to buy a lot of (serial) tickets (like 6 for 5 Zurich-Interlaken, 6 for 5 one-day passes in second and first class, one-dayclass chages from 2nd to 1sr class, etc. etc) on december-11 2004 (a day before the upcoming price increase).

These tickets are valid 3 years.