Run systems in with a fixed timestep

Use the FixedUpdate schedule.

fn main() {
  App::new()
    .add_systems(FixedUpdate, hello_world)
}

The FixedUpdate schedule will keep track of the total time elapsed and run your system after enough time has accumulated.

This can cause some issues during testing. Simply having your world update itself will not advance the time this schedule keeps track of. So in many cases during tests rescheduling your systems to run on update can be helpful.