We found ourselves spending New Year's eve heading to our friends place in Amstel. We took the Metro from Nieuwmarkt and the guard was already at the machine helping everyone out with their tickets. We'd done it many times before so we started doing our usual purchase: 4 x 1 Hour tickets (one each for the outbound journey and one each for the return trip since it would be more than an hour before we'd return).
The guard then told us that the Metro would be closed 8pm till 1am, which we were surprised to learn, and he also told that we only needed to buy 1 ticket each as long as we checked out properly at Amstel... because the tickets last for an hour of usage, NOT an hour from when they were purchased... so we've thrown away a dozen or so usable cards over the last year! Ah well, at least we found out now rather than after another year of living here!