Time is paused while you tinker with design of a new ship, and only begins when you click “launch,” meaning every ship is built instantly. Every second of spaceflight is added to the overall game clock, which tells the game where every planet and ship is in space. One of the quirks of KSP is that it keeps track of time. Simulate a few times, then when you’re happy with it, build it for real. Don’t forget: You can “simulate” a ship to see how it flies without waiting for the actual construction time.
