When you go to start an anctivity, the requirements for gold/silver/bronze are misleading in that they show you how much time you need to complete it in, not how much time you have left...
Example: You start that activity with 7:30 to complete it, so if you don't finish it faster than 3:30 (4:00 on the timer) you fail.
At least, that's how I understood it to be.