Timer Controlled Events

I’m creating a small flash file with a sunrise and sunset. Now, I want these to happen every 30 seconds but I can’t, for the life of me, work out how to do this using a timer function. Has anyone got any ideas?

What I need is after the first 30 seconds then the variable timeSunset = 1 and a further 30 seconds after that makes the variable timeSunrise = 1. This process needs to be kept going around in a loop with a 30 second gap between sunrise and sunset.