A precise method to measure how long you have been on.
This is for the Perfect Attendance Event.
Let's face it, the "You have played for X hours" is not an accurate measure of how long you have played. If I played for 5 minutes on one character, quickly moved onto the next, I would have only played for 55 minutes when the notice, "You have played for one hour" popped up. The timer, to my knowledge, does not reset if one switches accounts too quickly.
And here's a sure-fire method: The traits - Willpower, to be specific.
For every hour you play on a character, you gain a certain amount of Willpower. Simply check your Willpower through the Professions tab before you start waiting, and check again in about an hour. If you have gained Willpower, you have stayed your hour, and you can log off. If not, you will need to wait a little longer.