diff --git a/‎.changeset/20760-bucket-key-four-digit-year.md‎
Lines changed: 12 additions & 0 deletions b/‎.changeset/20760-bucket-key-four-digit-year.md‎
Lines changed: 12 additions & 0 deletions
diff --git a/‎packages/core/src/utils/datetime-bucket-key-four-digit-year.test.ts‎
Lines changed: 112 additions & 0 deletions b/‎packages/core/src/utils/datetime-bucket-key-four-digit-year.test.ts‎
Lines changed: 112 additions & 0 deletions
diff --git a/‎packages/core/src/utils/datetime-year-below-100.test.ts‎
Lines changed: 7 additions & 7 deletions b/‎packages/core/src/utils/datetime-year-below-100.test.ts‎
Lines changed: 7 additions & 7 deletions
diff --git a/‎packages/core/src/utils/datetime.ts‎
Lines changed: 51 additions & 10 deletions b/‎packages/core/src/utils/datetime.ts‎
Lines changed: 51 additions & 10 deletions
diff --git a/‎packages/drivers/driver-memory/src/memory-analytics-four-digit-year.test.ts‎
Lines changed: 65 additions & 0 deletions b/‎packages/drivers/driver-memory/src/memory-analytics-four-digit-year.test.ts‎
Lines changed: 65 additions & 0 deletions
@@ -0,0 +1,12 @@
+---
+'@objectstack/core': patch
+'@objectstack/service-analytics': patch
+---
+
+A date-bucket key spells its year with four digits at every granularity, as the SQL drivers' bucket expressions do, so the in-memory and pushed-down paths key a day in 0001..0999 alike and a drill-down from such a key finds its range.
+
+A `date` value names a year from 0001 to 9999, so these keys are reachable through a `date` field and through a stored `datetime` row. For 0050-06-15, `strftime('%Y-%m')` on SQLite and `to_char(…, 'YYYY-MM')` on PostgreSQL answer `0050-06`, while `bucketDateKey` answered `50-06`: the same `groupBy` keyed the same rows differently depending on which path ran it.
+
+- **`@objectstack/core` `bucketDateKey`** pads the year to four digits: `0050`, `0050-Q2`, `0050-06`, `0050-06-15`, and the ISO week key `0050-W24` (early January 0050 is `0049-W52`, its ISO week-year). The engine's in-memory `groupBy` and the memory cube face delegate to it, so both now answer the drivers' key. A year from 1000 to 9999 is spelled as before.
+- **`@objectstack/core` `bucketKeyToCalendarRange`** reads exactly what `bucketDateKey` writes. Its week arm checked a key against the unpadded label, so a padded key such as `0050-W01` answered `null`; it now answers `{ start: '0050-01-03', end: '0050-01-10' }`. An unpadded key (`50-06`, `49-W52`) is not a bucket key and still answers `null`.
+- **`@objectstack/service-analytics`** mints the `compareTo` alignment key through `bucketDateKey` instead of spelling it locally, so a comparison row in 0001..0999 merges onto its bucket (`0050-06`) instead of being appended under `50-06`.
@@ -0,0 +1,112 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// [#20760] A bucket key spells its year with four digits at every granularity,
+// as the drivers' bucket expressions do (`strftime('%Y')` on SQLite), and
+// `bucketKeyToCalendarRange` reads exactly what `bucketDateKey` writes.
+//
+// A `date` value keeps the years 0001..9999, so a key in 0001..0999 is
+// reachable through a `date` field and through a stored `datetime` row. Before
+// this card the writer spelled such a year unpadded (`50-06`, `49-W52`) while
+// SQL answered `0050-06`, and the reader's week arm checked a padded key
+// against the unpadded label, so `0050-W01` found no range.
+//
+// Pins: 0001, 0050 and 0999 at every granularity, the ISO week-year at a year
+// boundary (early January 0050 is in `0049-W52`), a drill-down from
+// `0050-W01`, and a 2026 control. Every expected key is spelled literally,
+// never computed by the code under test.
+
+import { describe, it, expect } from 'vitest';
+import { bucketDateKey, bucketKeyToCalendarRange, type BucketGranularity } from './datetime.js';
+
+const GRANULARITIES = ['year', 'quarter', 'month', 'day', 'week'] as const satisfies readonly BucketGranularity[];
+
+/** instant → the key at year / quarter / month / day / week. */
+const KEYS: ReadonlyArray<readonly [string, Record<BucketGranularity, string>]> = [
+  ['0001-01-01T10:00:00.000Z', { year: '0001', quarter: '0001-Q1', month: '0001-01', day: '0001-01-01', week: '0001-W01' }],
+  ['0050-06-15T10:00:00.000Z', { year: '0050', quarter: '0050-Q2', month: '0050-06', day: '0050-06-15', week: '0050-W24' }],
+  // The ISO week-numbering year is the previous calendar year here.
+  ['0050-01-01T10:00:00.000Z', { year: '0050', quarter: '0050-Q1', month: '0050-01', day: '0050-01-01', week: '0049-W52' }],
+  ['0999-06-15T10:00:00.000Z', { year: '0999', quarter: '0999-Q2', month: '0999-06', day: '0999-06-15', week: '0999-W24' }],
+  // The control: a four-digit year is spelled as it always was.
+  ['2026-06-15T10:00:00.000Z', { year: '2026', quarter: '2026-Q2', month: '2026-06', day: '2026-06-15', week: '2026-W25' }],
+];
+
+describe('[#20760] bucketDateKey spells the year with four digits', () => {
+  describe.each(KEYS)('%s', (instant, expected) => {
+    it.each(GRANULARITIES)('at %s', (g) => {
+      expect(bucketDateKey(instant, g)).toBe(expected[g]);
+      // A `Date` and epoch milliseconds name the same instant and get the same key.
+      expect(bucketDateKey(new Date(instant), g)).toBe(expected[g]);
+      expect(bucketDateKey(Date.parse(instant), g)).toBe(expected[g]);
+    });
+  });
+
+  it('a `date` value, the bare `YYYY-MM-DD` form, keys like its midnight', () => {
+    expect(bucketDateKey('0050-06-15', 'day')).toBe('0050-06-15');
+    expect(bucketDateKey('0050-06-15', 'month')).toBe('0050-06');
+    expect(bucketDateKey('0999-12-31', 'year')).toBe('0999');
+  });
+
+  it('the week year of a reference zone is four digits too', () => {
+    // Sunday 0050-01-02 in UTC is Monday 0050-01-03 in Shanghai: week 1 of 0050.
+    const instant = '0050-01-02T20:00:00.000Z';
+    expect(bucketDateKey(instant, 'week')).toBe('0049-W52');
+    expect(bucketDateKey(instant, 'week', 'Asia/Shanghai')).toBe('0050-W01');
+    expect(bucketDateKey(instant, 'day', 'Asia/Shanghai')).toBe('0050-01-03');
+  });
+});
+
+describe('[#20760] bucketKeyToCalendarRange reads exactly what bucketDateKey writes', () => {
+  describe.each(KEYS)('the keys of %s', (instant, expected) => {
+    it.each(GRANULARITIES)('at %s span a range whose first day keys back to it', (g) => {
+      const range = bucketKeyToCalendarRange(expected[g], g);
+      expect(range).not.toBeNull();
+      expect(bucketDateKey(range!.start, g)).toBe(expected[g]);
+      expect(range!.start <= instant.slice(0, 10) && instant.slice(0, 10) < range!.end).toBe(true);
+    });
+  });
+
+  it.each([
+    // A drill-down from the padded week key a SQL driver answers.
+    ['0050-W01', '0050-01-03', '0050-01-10'],
+    // The ISO week-year boundary: the week runs into January 0050.
+    ['0049-W52', '0049-12-27', '0050-01-03'],
+    ['0999-W24', '0999-06-10', '0999-06-17'],
+    // The control.
+    ['2026-W01', '2025-12-29', '2026-01-05'],
+  ])('week %s is %s up to %s', (key, start, end) => {
+    expect(bucketKeyToCalendarRange(key, 'week')).toEqual({ start, end });
+  });
+
+  it.each([
+    ['50', 'year'],
+    ['50-Q2', 'quarter'],
+    ['50-06', 'month'],
+    ['50-06-15', 'day'],
+    ['49-W52', 'week'],
+    ['999-W24', 'week'],
+  ] as const)('does not read the unpadded spelling %s (%s): nothing writes it', (key, g) => {
+    expect(bucketKeyToCalendarRange(key, g)).toBeNull();
+  });
+});
+
+describe('[#20760] a year outside 0000..9999 has no four-digit form, and is not padded into one', () => {
+  // Not reached: at both engine doors a `date` value names a year from 0001 to
+  // 9999 and a `datetime` one a year from 1000 to 9999. Stated so a negative
+  // year is never spelled as a padded fragment (`00-1`) that reads as a key.
+  const inYear = (year: number) => {
+    const d = new Date(0);
+    d.setUTCFullYear(year, 5, 15);
+    return d;
+  };
+
+  it.each([
+    [-1, '-1', '-1-06-15'],
+    [10000, '10000', '10000-06-15'],
+  ])('year %i keys as %s and %s, and no range reads it', (year, yearKey, dayKey) => {
+    expect(bucketDateKey(inYear(year), 'year')).toBe(yearKey);
+    expect(bucketDateKey(inYear(year), 'day')).toBe(dayKey);
+    expect(bucketKeyToCalendarRange(yearKey, 'year')).toBeNull();
+    expect(bucketKeyToCalendarRange(dayKey, 'day')).toBeNull();
+  });
+});
@@ -158,11 +158,12 @@ describe.each(HOSTS)('on a %s host', (host) => {
   });
 
   describe('[#20599] bucketDateKey(week) puts a day in 0001..0099 in its own ISO week', () => {
-    // A key's year below 1000 is spelled unpadded (`49-W52`): the unpadded-key
-    // family, which this card leaves alone. So the key is read as numbers here,
-    // whatever its padding, and asserted as the ISO week of the day.
+    // [#20760] The key's year is four digits (`0049-W52`), so the key is read
+    // as a four-digit year and a week, and asserted as the ISO week of the
+    // day; an unpadded key does not parse and fails the assertion. The
+    // spelling itself is pinned in `datetime-bucket-key-four-digit-year.test.ts`.
     const weekOf = (key: string | null) => {
-      const m = /^(\d+)-W(\d{2})$/.exec(String(key));
+      const m = /^(\d{4})-W(\d{2})$/.exec(String(key));
       return m ? { year: Number(m[1]), week: Number(m[2]) } : key;
     };
 
@@ -242,9 +243,8 @@ describe.each(HOSTS)('on a %s host', (host) => {
       expect(bucketKeyToCalendarRange('2026-02-29', 'day')).toBeNull();
     });
 
-    // The week arm validates a reconstructed Monday against the week LABEL,
-    // which is spelled unpadded below the year 1000 (the unpadded-key family,
-    // left alone here), so it is pinned on the 2026 control only.
+    // [#20760] The week arm's keys in 0001..0999 (`0050-W01`, `0049-W52`) are
+    // pinned in `datetime-bucket-key-four-digit-year.test.ts`.
     it('week 2026-W01 (the control)', () => {
       expect(bucketKeyToCalendarRange('2026-W01', 'week')).toEqual({ start: '2025-12-29', end: '2026-01-05' });
     });
 
@@ -302,6 +302,13 @@ export function isBucketGranularity(value: unknown): value is BucketGranularity
  * Null and unparseable deliberately share one bucket: SQL cannot tell them apart
  * either (`strftime('%Y-%m', 'not-a-date')` is NULL), and splitting them here
  * would re-open the seam this function exists to close.
+ *
+ * [#20760] The year of every key is spelled with four digits
+ * ({@link bucketKeyYear}): `0050`, `0050-Q2`, `0050-06`, `0050-06-15`,
+ * `0049-W52` — what the drivers' bucket expressions answer for the same
+ * instant. A `date` value keeps the years 0001..9999, and a `datetime` row
+ * stored before the engine doors refused a year below 1000 can still hold one,
+ * so 0001..0999 reach this function.
  */
 export function bucketDateKey(
   value: unknown,
@@ -319,13 +326,13 @@ export function bucketDateKey(
   const { year: y, month: m, day } = calendarPartsInTzOrUtc(d, timezone);
   switch (granularity) {
     case 'year':
-      return String(y);
+      return bucketKeyYear(y);
     case 'quarter':
-      return `${y}-Q${Math.floor((m - 1) / 3) + 1}`;
+      return `${bucketKeyYear(y)}-Q${Math.floor((m - 1) / 3) + 1}`;
     case 'month':
-      return `${y}-${String(m).padStart(2, '0')}`;
+      return `${bucketKeyYear(y)}-${String(m).padStart(2, '0')}`;
     case 'day':
-      return `${y}-${String(m).padStart(2, '0')}-${String(day).padStart(2, '0')}`;
+      return bucketDayKey(y, m, day);
     case 'week':
       return isoWeekLabelFromCalendarDay(y, m, day);
     default:
@@ -337,6 +344,33 @@ export function bucketDateKey(
   }
 }
 
+/**
+ * [#20760] The year of a bucket key, spelled with four digits: `50` is
+ * `0050`, `999` is `0999`, `2026` is `2026`.
+ *
+ * The ONE statement of the key's year spelling. Every key
+ * {@link bucketDateKey} writes, the ISO week label, and the calendar bounds
+ * {@link bucketKeyToCalendarRange} answers spell their year through it, so
+ * the writer and the reader cannot disagree about it. It is the width the
+ * drivers' bucket expressions answer (`strftime('%Y')` on SQLite, `YYYY` /
+ * `IYYY` in PostgreSQL's `to_char`, `%Y` / `%x` in MySQL's `date_format`),
+ * and `bucketDateKey`'s contract is that its label equals theirs.
+ *
+ * A year below 0 has no four-digit form and keeps its plain spelling (`-1`),
+ * never a padded fragment such as `00-1`; a year past 9999 is longer than four
+ * digits already. Neither is reached: at both engine doors a `date` value
+ * names a year from 0001 to 9999 and a `datetime` one a year from 1000 to
+ * 9999, and {@link bucketKeyToCalendarRange} reads neither spelling.
+ */
+function bucketKeyYear(year: number): string {
+  return year >= 0 ? String(year).padStart(4, '0') : String(year);
+}
+
+/** [#20760] The `day` key (`YYYY-MM-DD`) of a calendar day, `month` 1-12. */
+function bucketDayKey(year: number, month: number, day: number): string {
+  return `${bucketKeyYear(year)}-${String(month).padStart(2, '0')}-${String(day).padStart(2, '0')}`;
+}
+
 /**
  * ISO-8601 week label (Mon-start weeks, week 1 = the week of the first
  * Thursday) of a calendar day given that day's parts (`month` is 1-12).
@@ -347,6 +381,10 @@ export function bucketDateKey(
  *
  * [#20599] Both days are built by {@link wallClockToUtcMs}, so a day in
  * 0001..0099 lands in its own ISO week, never in the 1900s one.
+ *
+ * [#20760] The label's year is the ISO week-numbering year, spelled with four
+ * digits ({@link bucketKeyYear}). Early in January it can be the previous
+ * calendar year: 0050-01-01 is in `0049-W52`.
  */
 function isoWeekLabelFromCalendarDay(year: number, month: number, day: number): string {
   const target = new Date(wallClockToUtcMs({ year, month, day }));
@@ -361,7 +399,7 @@ function isoWeekLabelFromCalendarDay(year: number, month: number, day: number):
         ((firstThursday.getUTCDay() + 6) % 7)) /
         7,
     );
-  return `${target.getUTCFullYear()}-W${String(weekNo).padStart(2, '0')}`;
+  return `${bucketKeyYear(target.getUTCFullYear())}-W${String(weekNo).padStart(2, '0')}`;
 }
 
 /**
@@ -398,17 +436,20 @@ function isoWeekLabelUtc(d: Date): string {
  * never the 1900s one. The arms lean on its rollover, which is `Date.UTC`'s:
  * month 13 is next January (Q4's and December's end), and day 32 the next
  * month.
+ *
+ * [#20760] It reads exactly what {@link bucketDateKey} writes: a four-digit
+ * year at every granularity, the week key included (`0050-W01`). The day and
+ * week arms check a key against the label the writer gives the reconstructed
+ * day, and every bound is spelled by the writer's own day key, so the reader
+ * cannot drift from the writer. An unpadded key (`50-06`, `49-W52`) is not a
+ * bucket key and answers `null`.
  */
 export function bucketKeyToCalendarRange(
   key: string | null | undefined,
   granularity: BucketGranularity,
 ): { start: string; end: string } | null {
   if (typeof key !== 'string' || key.length === 0) return null;
-  const fmt = (dt: Date) =>
-    `${String(dt.getUTCFullYear()).padStart(4, '0')}-${String(dt.getUTCMonth() + 1).padStart(
-      2,
-      '0',
-    )}-${String(dt.getUTCDate()).padStart(2, '0')}`;
+  const fmt = (dt: Date) => bucketDayKey(dt.getUTCFullYear(), dt.getUTCMonth() + 1, dt.getUTCDate());
   /** Midnight UTC of `year`-`month`-`day`, `month` 1-12, rolled over past its end. */
   const utcDay = (year: number, month: number, day: number) =>
     new Date(wallClockToUtcMs({ year, month, day }));
 
@@ -0,0 +1,65 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// [#20760] The memory cube face labels a time bucket in 0001..0999 with a
+// four-digit year — `0050`, `0050-Q2`, `0050-06`, `0050-06-15`, `0050-W24` —
+// the key a SQL driver's bucket expression answers for the same instant. It
+// folds the key through `@objectstack/core`'s `bucketDateKey`; this pins the
+// face. Before the card it answered `50`, `50-Q2`, `50-06`, `50-06-15` and
+// `50-W24`, and a drill-down from such a key found no range.
+
+import { describe, it, expect } from 'vitest';
+import { InMemoryDriver } from './memory-driver.js';
+import { MemoryAnalyticsService } from './memory-analytics.js';
+import { AnalyticsQuerySchema } from '@objectstack/spec/data';
+import type { AnalyticsQuery, Cube } from '@objectstack/spec/data';
+
+const cubes: Cube[] = [
+  {
+    name: 'events',
+    title: 'Events',
+    sql: 'events',
+    measures: { count: { label: 'Event Count', type: 'count', sql: 'id' } },
+    dimensions: {
+      createdAt: {
+        label: 'Created At',
+        type: 'time',
+        sql: 'created_at',
+        granularities: ['day', 'week', 'month', 'quarter', 'year'],
+      },
+    },
+  },
+];
+
+const ROWS = [
+  { id: 1, created_at: '0050-06-15T10:00:00.000Z' },
+  { id: 2, created_at: '0999-06-15T10:00:00.000Z' },
+  // The control.
+  { id: 3, created_at: '2026-06-15T10:00:00.000Z' },
+];
+
+async function labels(granularity: 'day' | 'week' | 'month' | 'quarter' | 'year'): Promise<unknown[]> {
+  const driver = new InMemoryDriver({ initialData: { events: ROWS } });
+  await driver.connect();
+  const service = new MemoryAnalyticsService({ driver, cubes });
+  const result = await service.query(
+    AnalyticsQuerySchema.parse({
+      cube: 'events',
+      measures: ['events.count'],
+      dimensions: ['events.createdAt'],
+      timeDimensions: [{ dimension: 'events.createdAt', granularity }],
+    } satisfies AnalyticsQuery),
+  );
+  return result.rows.map((r) => r['events.createdAt']).sort();
+}
+
+describe('[#20760] the memory cube face labels a year below 1000 with four digits', () => {
+  it.each([
+    ['year', ['0050', '0999', '2026']],
+    ['quarter', ['0050-Q2', '0999-Q2', '2026-Q2']],
+    ['month', ['0050-06', '0999-06', '2026-06']],
+    ['day', ['0050-06-15', '0999-06-15', '2026-06-15']],
+    ['week', ['0050-W24', '0999-W24', '2026-W25']],
+  ] as const)('at %s', async (granularity, expected) => {
+    expect(await labels(granularity)).toEqual(expected);
+  });
+});