A spaceship is flying a distance of 5 light hours, for example from Earth to the dwarf planet Pluto. The speed can be regulated with the upper buttons. The app demonstrates that the clock in the spaceship goes more slowly than the two clocks of the system in which Earth and Pluto are motionless.
t' ... time indicated by the spaceship clock
t ... time indicated by the clocks of the Earth-Pluto-system
v ... speed of the spacecraft relatively to the system of Earth and Pluto
c ... speed of light