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+# Date observability
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+The Date formatter exports two OpenTelemetry instruments and a small
+in-memory state ring so spec-conformance tests can assert exact
+attribute payloads without standing up a full SDK exporter. The
+instrument names match the canonical Wave-3 formatter contract —
+every sibling formatter (number, relative-time, list, display-name,
+currency, collator, segmenter) emits the same two metric names so
+dashboards and alerts compose cleanly across the family.
+
+## Features
+
+- Histogram `i18n.format.date.duration_ms` (unit `ms`) — per-format wall-clock time
+- Counter `i18n.format.cache_hit{formatter="date"}` — increments on cache reuse only
+- In-memory state ring (`getDateMetricsState()`) for spec-conformance
+- `resetDateMetricsState()` test-only helper to clear the ring
+
+## Usage: Basic Example
+
+```ts
+import { metrics } from "@opentelemetry/api";
+import { MeterProvider } from "@opentelemetry/sdk-metrics";
+
+const provider = new MeterProvider({ readers: [/* reader */] });
+metrics.setGlobalMeterProvider(provider);
+
+import { formatDate } from "@suaveplan/i18n-formatter-date";
+formatDate(new Date(2026, 0, 1), "en-GB", { preset: "short" });
+// emits via your reader
+```
+
+## Usage: Advanced Example
+
+```ts
+import { describe, expect, it } from "@suaveplan/testing/runner";
+import {
+    clearDateFormatterCache,
+    formatDate,
+} from "@suaveplan/i18n-formatter-date";
+import {
+    getDateMetricsState,
+    resetDateMetricsState,
+} from "@suaveplan/i18n-formatter-date";
+
+it("emits histogram + cache-hit counter", () => {
+    clearDateFormatterCache();
+    resetDateMetricsState();
+    formatDate(new Date(2026, 0, 1), "en-GB", { preset: "short" });
+    formatDate(new Date(2026, 0, 1), "en-GB", { preset: "short" });
+    const state = getDateMetricsState();
+    expect(state.durationSamples).toHaveLength(2);
+    expect(state.cacheHitSamples).toHaveLength(1);
+    expect(state.durationSamples[0]?.attributes).toMatchObject({
+        locale: "en-GB",
+        preset: "short",
+    });
+});
+```
+
+## API Reference
+
+### `recordDateFormat(locale, preset, durationMs): void`
+
+Records the duration histogram sample. Tags: `locale`, `preset`.
+Called internally by `formatDate` / `formatDateRange`.
+
+### `recordDateCacheHit(): void`
+
+Increments the cache-hit counter with the constant tag
+`{ formatter: "date" }`.
+
+### `getDateMetricsState(): DateMetricsState`
+
+Returns the in-memory ring of `durationSamples` and
+`cacheHitSamples`. Used by spec-conformance tests.
+
+### `resetDateMetricsState(): void`
+
+Clears both ring buffers. Test-only helper.
+
+## Implementation Notes
+
+- The OTel meter is named `suaveplan.i18n.formatter.date`. Every
+  formatter in the family has its own meter so OTel resource
+  attributes can be tied per-package if a consumer chooses.
+- When the caller omits `preset`, the histogram tag is `"custom"` so
+  every sample carries a non-null preset attribute.
+- Sample dashboard query (PromQL/Grafana over OTLP):
+  `histogram_quantile(0.99, sum(rate(i18n_format_date_duration_ms_bucket[5m])) by (le, locale, preset))`
+- The in-memory ring is intentionally unbounded — tests are short
+  and call `resetDateMetricsState()` between cases. A long-running
+  test loop should call reset to avoid memory growth.