Function ms_from_steps 

pub const fn ms_from_steps(steps: u32) -> u64

The milliseconds steps steps last at STEPS_PER_SECOND, rounded down: 0, 16, 33, 50, … and exactly 1000 at step 60, so it never drifts as a whole number of milliseconds per step would. steps_from_ms of it gives steps back while that fits a u32.

It is the engine’s one rule from steps to milliseconds: data timed in milliseconds (a clip, a particle’s life, a bone clip) is read at ms_from_steps(step - start_step), by frame_at, particles_at and pose_at alike.