diff --git a/docs/agent-guides/domain-invariants.md b/docs/agent-guides/domain-invariants.md
index 01fe8df35..9418e4901 100644
--- a/docs/agent-guides/domain-invariants.md
+++ b/docs/agent-guides/domain-invariants.md
@@ -22,6 +22,10 @@ Load only for touched domains. Add only stable, recurring invariants; not one-of
 
 ## Timers
 
+Use temporal values by meaning: `Instant` for epoch time, `TimeOfDay` for local time since midnight, `Duration` for elapsed time, `Day` for calendar offsets. Convert through `timeCore`; never interchange as raw numbers.
+
+Active work: [runtimeState time-core migration](../migrations/runtime-state-time-core.md).
+
 When relevant, cover interactions among:
 
 - midnight/day offsets;
diff --git a/docs/migrations/runtime-state-time-core.md b/docs/migrations/runtime-state-time-core.md
new file mode 100644
index 000000000..27c38df47
--- /dev/null
+++ b/docs/migrations/runtime-state-time-core.md
@@ -0,0 +1,67 @@
+# runtimeState time-core migration
+
+**Status:** In progress
+
+## Goal
+
+Make temporal meaning explicit across `runtimeState` and timer calculations. Prevent mixing:
+
+- `Instant`: epoch timestamp;
+- `TimeOfDay`: local milliseconds since midnight, range `[0, dayInMs)`;
+- `Duration`: elapsed or remaining milliseconds;
+- `Day`: calendar-day offset.
+
+Use `apps/server/src/lib/time-core/timeCore.ts` for conversions and temporal arithmetic. Branded types remain numbers at serialization boundaries.
+
+## Current state
+
+Completed foundation:
+
+- temporal brands in `ontime-types`;
+- `timeCore` helpers for now, conversion, duration arithmetic, midnight crossing, calendar-day distance;
+- partial `runtimeState` adoption of `Instant`, `TimeOfDay`, `Day`, and `timeCore`.
+
+Remaining ambiguity:
+
+- `TimerState`, `RundownState`, and `Offset` expose temporal fields as `number`/`MaybeNumber`;
+- `timerUtils` accepts/returns raw numbers with different meanings;
+- `runtimeState` retains raw duration fields, manual arithmetic, and `as TimeOfDay`/`as Duration` casts.
+
+## Migration rules
+
+- Classify each temporal field before changing it. No generic `Time` type.
+- Convert only through `timeCore` or an explicit transport adapter.
+- Keep public/websocket JSON numeric where required; brand at the boundary.
+- Keep type migration separate from behaviour changes.
+- Preserve current midnight, rollover, pause, add-time, roll, and offset behaviour per slice.
+- Add helpers only when they encode a named temporal rule and remove caller casts/arithmetic.
+
+## Slices
+
+- [x] Add temporal brands and initial `timeCore` helpers/tests.
+- [ ] Inventory temporal fields in `TimerState`, `RundownState`, `Offset`, and private runtime state; assign intended types.
+- [ ] Migrate pure `timerUtils` functions by temporal concept; add focused midnight/rollover tests.
+- [ ] Migrate private `RuntimeState` fields and calculations; remove local casts/manual conversions.
+- [ ] Migrate shared runtime contracts and add numeric transport adapters where compatibility requires them.
+- [ ] Update remaining callers, fixtures, and mocks.
+- [ ] Remove superseded helpers, casts, and ambiguous temporal numbers.
+
+Each slice must leave old/new boundaries explicit, compile cleanly, and preserve behaviour.
+
+## Required coverage
+
+- before, at, and after midnight;
+- overnight and multi-day rundowns;
+- local-day calculation across timezone/DST offset changes;
+- pause/resume, added time, elapsed/remaining duration;
+- roll secondary targets and expected finish;
+- absolute/relative offsets and day offsets.
+
+## Complete when
+
+- Runtime/timer boundaries use semantic temporal types or documented numeric transport fields.
+- Temporal conversions/arithmetic use `timeCore` or named pure helpers.
+- No unexplained temporal casts or ambiguous numeric fields remain in migrated scope.
+- Focused tests cover the required boundaries.
+
+After completion, remove this migration tracker. Keep durable rules in `docs/agent-guides/domain-invariants.md`.