A virtual clock has to be the only clock, or what it drives never moves

Fact

Adding a way to step time — a test hook, a replay, a timeline scrubber — introduces a second clock. Anything that still calls performance.now() directly is stamping events on the real timeline while the stepper advances a different one, and the two drift apart immediately.

The usual result is an animation that never starts. A tween records start = performance.now(), the stepper advances from a baseline captured before that, elapsed time comes out negative, and the obligatory clamp(elapsed / duration, 0, 1) pins progress at zero. Stepping further does not help; the baseline is behind the start and stays behind it.

Why it matters

Nothing fails. The tween object exists, the step call returns, the frame is drawn, and the state does not change — so it reads as "the animation was never wired up" and sends you into the code that creates it, which is correct.

The opposite drift is worse, because it half-works: when the stepper is ahead of the recorded start, progress clamps to 1 on the first step, the animation completes instantly, and the final state is right. Every assertion about the outcome passes and only the motion is missing, so the harness certifies an animation it never actually ran.

How to apply

Express the virtual clock as an offset on the real one and route every reader through it, the code that records start times included:
```
let offset = 0;
const clock = () => performance.now() + offset;
const step = (ms) => { offset += ms; advance(clock()); render(); };
```
A stepped clock and a real frame then cannot disagree about "now".
Step by moving the offset, not by passing a chosen timestamp into advance. A caller-supplied timestamp is a second source of truth and will diverge.
After adding the hook, grep for remaining direct performance.now() and Date.now() calls. The ones you miss are precisely the bug.
Progress stuck at 0, or jumping straight to 1, is the signature — a genuinely wrong animation moves, it just moves wrongly.

Related: [[driven-game-loop-needs-accumulator-reset]], [[animation-lifetime-in-frames]], [[hidden-surface-delivers-no-frames]].