Clock starts from when you first validate the card.

If you arrive at the Central rail station, don't buy the ticket from the tourist/GVB office directly outside (can be very long queues) but go down to the underground station just below where there's unlikely to be anyone one waiting.

These tickets are definitely not good on NS (Nederland Spoorwagen or whatever) trains or any non-GVB transport (see next paragraph). At least not last June. Only worth buying from/at AMS if you are using the bus to get into the city and I'm not sure if you can buy it there.

Be aware that they're not good for any buses that run slightly outside Amsterdam (including Amsterveen in the south). Only on transit operated by GVB.