Using time as an input in Simulink

I am currently doing a modeling about sun position. Since an equation has a time as input, is it possible for me to set a start time and end time for it to run? For example, I set 9am for the starting time and end it on 5pm. The model will recalculate the sun position every 15 minutes, it is possible to do that? What should I add to achieve this? Thank you.