A simulation's scheduled events must use its own accumulated clock, not wall-clock deadlines

Fact

A simulation that advances continuous state (x += rate * dt) usually also has one-off events on timers — spawn something, roll for an event, fire a cooldown. It is natural to store those deadlines as absolute timestamps:

pet.nextSpawnAt = Date.now() + 90000;      // wall clock
if (now >= pet.nextSpawnAt) { spawn(); }

That puts the two halves of the simulation in different clock domains. The continuous half lives in accumulated simulated time; the event half lives in real time. They agree only while the simulation is stepped in lockstep with the wall clock — which is exactly the case that stops holding as soon as anything drives it: a test that advances the clock, a replay, an offline catch-up run in coarse steps, a scrubbed timeline, a speed multiplier.

The continuous state then races ahead while the deadlines stay where the wall clock left them, and no scheduled event ever fires.

Why it matters

Nothing throws and nothing logs. The simulation visibly runs — stats move, things age, rendering continues — so the missing half looks like a bug in the event logic rather than a missing clock. Investigation goes to the spawn condition, the probability, the guard clauses, all of which are correct.

It is worst in a test harness, because the harness is the thing that drives the clock. A suite that advances a day of simulated time and asserts "no random events occurred" passes for the wrong reason, and keeps passing.

How to apply

Keep one clock. Store deadlines in the simulation's own accumulated time — the same quantity the continuous state integrates:
```
pet.nextSpawnDay = pet.ageDays + 90 / SECONDS_PER_DAY;
if (pet.ageDays >= pet.nextSpawnDay) { spawn(); }
```
Wall-clock timestamps still belong at the boundary: one lastTs recording the last simulated moment, used to work out how much time to advance.
The tell is a driven run where continuous state changes and discrete events never fire. Assert on both, not just on the numbers that move.

Related: [[driven-game-loop-needs-accumulator-reset]], [[animation-lifetime-in-frames]].