Exactly, the action is "tick" the model then decide how the system responds to a tick. The separation Action/Model provides a boundary that isolates the system (model) from knowledge of the outside. Let's take the example of leap seconds. Incidentally both the American and the French question why we have to align time so precisely with the revolution of the Earth around the Sun, and you guess it, the Brits are for that alignment (are you guys still navigating with Sextants?). But I degress, what would be the best way to implement the leap second? an action or the model? The model should implement the clock, translating ticks into a readable time. The clock should know nothing about how the ticks are generated (from grain of sands to quartz vibrations). Should the clock know anything about leap seconds? probably not, a sensible implementation would be an action that receives a tick event, queries the current position of the earth and decides to increment by two, one or zero depending if the clock is ahead, at or behind the expected position of the earth.