diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..e607628 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,69 @@
  * calendar date in 'YYYY-MM-DD' form.
  */
 export function billingDates(start, rule) {
-  throw new Error('not implemented');
+  if (rule === null || typeof rule !== 'object') throw new RangeError('rule must be an object');
+  const { every, interval = 1, count, until, skip = [] } = rule;
+
+  if (every !== 'week' && every !== 'month') throw new RangeError(`invalid every: ${every}`);
+  if (!isPositiveInt(interval)) throw new RangeError(`invalid interval: ${interval}`);
+  const hasCount = count !== undefined && count !== null;
+  const hasUntil = until !== undefined && until !== null;
+  if (!hasCount && !hasUntil) throw new RangeError('count or until is required');
+  if (hasCount && !isPositiveInt(count)) throw new RangeError(`invalid count: ${count}`);
+
+  const s = parseDate(start, 'start');
+  const untilKey = hasUntil ? parseDate(until, 'until') && until : null;
+  const skipped = new Set(skip);
+
+  const result = [];
+  for (let k = 0; ; k++) {
+    const date = every === 'week' ? addDays(s, 7 * interval * k) : addMonths(s, interval * k);
+    const text = format(date);
+    // 'YYYY-MM-DD' strings compare chronologically.
+    if (untilKey !== null && text > untilKey) break;
+    if (!skipped.has(text)) {
+      result.push(text);
+      if (hasCount && result.length === count) break;
+    }
+  }
+  return result;
+}
+
+function isPositiveInt(n) {
+  return Number.isSafeInteger(n) && n > 0;
+}
+
+function parseDate(text, name) {
+  const m = typeof text === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(text);
+  if (!m) throw new RangeError(`invalid ${name}: ${text}`);
+  const [y, mo, d] = [Number(m[1]), Number(m[2]), Number(m[3])];
+  if (mo < 1 || mo > 12 || d < 1 || d > daysInMonth(y, mo)) {
+    throw new RangeError(`invalid ${name}: ${text}`);
+  }
+  return { y, mo, d };
+}
+
+function daysInMonth(y, mo) {
+  const leap = (y % 4 === 0 && y % 100 !== 0) || y % 400 === 0;
+  return [31, leap ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][mo - 1];
+}
+
+// Month arithmetic from the original start day, clamped to the month's length.
+function addMonths({ y, mo, d }, n) {
+  const idx = y * 12 + (mo - 1) + n;
+  const ny = Math.floor(idx / 12);
+  const nmo = (idx % 12) + 1;
+  return { y: ny, mo: nmo, d: Math.min(d, daysInMonth(ny, nmo)) };
+}
+
+function addDays({ y, mo, d }, n) {
+  // setUTCFullYear avoids Date.UTC's mapping of years 0-99 to 1900-1999.
+  const dt = new Date(0);
+  dt.setUTCFullYear(y, mo - 1, d + n);
+  return { y: dt.getUTCFullYear(), mo: dt.getUTCMonth() + 1, d: dt.getUTCDate() };
+}
+
+function format({ y, mo, d }) {
+  if (y > 9999) throw new RangeError('billing date beyond year 9999');
+  return `${String(y).padStart(4, '0')}-${String(mo).padStart(2, '0')}-${String(d).padStart(2, '0')}`;
 }
diff --git a/tests/schedule.test.js b/tests/schedule.test.js
index ee3d88f..bb74abd 100644
--- a/tests/schedule.test.js
+++ b/tests/schedule.test.js
@@ -5,3 +5,31 @@ import { billingDates } from '../src/schedule.js';
 test('monthly, mid-month start', () => {
   assert.deepEqual(billingDates('2026-01-15', { every: 'month', count: 3 }), ['2026-01-15', '2026-02-15', '2026-03-15']);
 });
+
+test('monthly clamps to month end and returns to original day', () => {
+  assert.deepEqual(billingDates('2026-01-31', { every: 'month', count: 4 }), ['2026-01-31', '2026-02-28', '2026-03-31', '2026-04-30']);
+  assert.deepEqual(billingDates('2024-01-31', { every: 'month', count: 2 }), ['2024-01-31', '2024-02-29']);
+});
+
+test('interval, weeks, until, skip', () => {
+  assert.deepEqual(billingDates('2026-11-30', { every: 'month', interval: 3, count: 3 }), ['2026-11-30', '2027-02-28', '2027-05-30']);
+  assert.deepEqual(billingDates('2026-12-28', { every: 'week', count: 3 }), ['2026-12-28', '2027-01-04', '2027-01-11']);
+  assert.deepEqual(billingDates('2026-01-01', { every: 'week', interval: 2, until: '2026-02-12' }), ['2026-01-01', '2026-01-15', '2026-01-29', '2026-02-12']);
+  assert.deepEqual(billingDates('2026-01-15', { every: 'month', count: 2, skip: ['2026-02-15'] }), ['2026-01-15', '2026-03-15']);
+  assert.deepEqual(billingDates('2026-01-15', { every: 'month', count: 5, until: '2026-03-15', skip: ['2026-01-15'] }), ['2026-02-15', '2026-03-15']);
+  assert.deepEqual(billingDates('2026-05-01', { every: 'month', until: '2026-04-01' }), []);
+});
+
+test('invalid input throws RangeError', () => {
+  const bad = [
+    ['2026-01-01', { every: 'month' }],
+    ['2026-01-01', { every: 'day', count: 1 }],
+    ['2026-01-01', { every: 'month', count: 0 }],
+    ['2026-01-01', { every: 'month', count: 1.5 }],
+    ['2026-01-01', { every: 'month', count: 1, interval: 0 }],
+    ['2026-02-30', { every: 'month', count: 1 }],
+    ['2026-1-01', { every: 'month', count: 1 }],
+    ['2026-01-01', { every: 'month', until: '2026-13-01' }],
+  ];
+  for (const [s, r] of bad) assert.throws(() => billingDates(s, r), RangeError);
+});