TWO DIFFERENT TIMER STYLES BELOW:
For the first timer, Click on the grey up and down arrows above the first World Boss picture, to see the waypoint code. Mouse click on the waypoint code, i.e. [&BM0CAAA=], to copy it. In game, you can paste the code in /say or /s chat to see the boss's waypoint.
Green Square means the boss is active right now.
Guild Wars 2 Wiki Event Timers (for any boss not found above):