I see that a single ticket from AMS to Amsterdam Centraal (or vice-versa) is 3.10 Euros and a return is 5.80 or something. My question is this: can I use a return on two different days, or must it be used on the same day?
"Everyone writing for the Telegraph knows that the way to grab eyeballs is with Ryanair and/or sex."
Your ticket is only valid on the day you buy it up to 04:00 next day in the morning. At that time most train services stop anyway except for a few routes (including Schiphol - Amsterdam if I am not mistaken).
If you buy your ticket at 01:00 in the morning you can use it for 27 hours. If you buy your ticket at 23:00 you can only use it for 5 hours.