diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..6d71c01 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,63 @@
  * calendar date in 'YYYY-MM-DD' form.
  */
 export function billingDates(start, rule) {
-  throw new Error('not implemented');
+  const { every, interval = 1, count, until, skip = [] } = rule ?? {};
+
+  if (every !== 'week' && every !== 'month') throw new RangeError(`every must be 'week' or 'month', got ${every}`);
+  if (!isPositiveInt(interval)) throw new RangeError(`interval must be a positive integer, got ${interval}`);
+  if (count == null && until == null) throw new RangeError('give count, until, or both');
+  if (count != null && !isPositiveInt(count)) throw new RangeError(`count must be a positive integer, got ${count}`);
+
+  const first = parseDate(start, 'start');
+  if (until != null) parseDate(until, 'until');
+  const skipped = new Set(skip);
+
+  const result = [];
+  for (let i = 0; count == null || result.length < count; i++) {
+    const date = every === 'week' ? addWeeks(first, i * interval) : addMonths(first, i * interval);
+    // 'YYYY-MM-DD' strings sort chronologically.
+    if (until != null && date > until) break;
+    if (!skipped.has(date)) result.push(date);
+  }
+  return result;
+}
+
+function isPositiveInt(n) {
+  return Number.isInteger(n) && n > 0;
+}
+
+function daysInMonth(year, month) {
+  // month is 1-12
+  if (month === 2) return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0) ? 29 : 28;
+  return [4, 6, 9, 11].includes(month) ? 30 : 31;
+}
+
+function parseDate(str, name) {
+  const m = typeof str === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(str);
+  if (m) {
+    const [year, month, day] = [Number(m[1]), Number(m[2]), Number(m[3])];
+    if (month >= 1 && month <= 12 && day >= 1 && day <= daysInMonth(year, month)) {
+      return { year, month, day };
+    }
+  }
+  throw new RangeError(`${name} must be a real calendar date in YYYY-MM-DD form, got ${str}`);
+}
+
+function format({ year, month, day }) {
+  const p = (n, w) => String(n).padStart(w, '0');
+  return `${p(year, 4)}-${p(month, 2)}-${p(day, 2)}`;
+}
+
+// Always computed from the start date, so a clamped day never carries into later months.
+function addMonths({ year, month, day }, months) {
+  const index = year * 12 + (month - 1) + months;
+  const y = Math.floor(index / 12);
+  const m = (index % 12) + 1;
+  return format({ year: y, month: m, day: Math.min(day, daysInMonth(y, m)) });
+}
+
+function addWeeks({ year, month, day }, weeks) {
+  const d = new Date(0);
+  d.setUTCFullYear(year, month - 1, day + weeks * 7);
+  return format({ year: d.getUTCFullYear(), month: d.getUTCMonth() + 1, day: d.getUTCDate() });
 }
diff --git a/tests/schedule.test.js b/tests/schedule.test.js
index ee3d88f..a71a2a6 100644
--- a/tests/schedule.test.js
+++ b/tests/schedule.test.js
@@ -2,6 +2,53 @@ import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { billingDates } from '../src/schedule.js';
 
+const m = (start, rule) => billingDates(start, { every: 'month', ...rule });
+
 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('month-end clamps then returns to original day', () => {
+  assert.deepEqual(m('2026-01-31', { count: 4 }), ['2026-01-31', '2026-02-28', '2026-03-31', '2026-04-30']);
+});
+
+test('leap year february', () => {
+  assert.deepEqual(m('2028-01-31', { count: 2 }), ['2028-01-31', '2028-02-29']);
+});
+
+test('monthly interval across year boundary', () => {
+  assert.deepEqual(m('2026-11-30', { interval: 3, count: 3 }), ['2026-11-30', '2027-02-28', '2027-05-30']);
+});
+
+test('weekly with interval, across month and year', () => {
+  assert.deepEqual(billingDates('2026-12-20', { every: 'week', interval: 2, count: 3 }), ['2026-12-20', '2027-01-03', '2027-01-17']);
+});
+
+test('until is inclusive and combines with count', () => {
+  assert.deepEqual(m('2026-01-15', { until: '2026-03-15' }), ['2026-01-15', '2026-02-15', '2026-03-15']);
+  assert.deepEqual(m('2026-01-15', { until: '2026-03-14' }), ['2026-01-15', '2026-02-15']);
+  assert.deepEqual(m('2026-01-15', { count: 2, until: '2026-12-15' }), ['2026-01-15', '2026-02-15']);
+  assert.deepEqual(m('2026-01-15', { until: '2026-01-01' }), []);
+});
+
+test('skipped dates are omitted and do not count', () => {
+  assert.deepEqual(m('2026-01-15', { count: 3, skip: ['2026-01-15', '2026-03-15'] }), ['2026-02-15', '2026-04-15', '2026-05-15']);
+  assert.deepEqual(m('2026-01-15', { until: '2026-03-15', skip: ['2026-02-15'] }), ['2026-01-15', '2026-03-15']);
+});
+
+test('invalid input throws RangeError', () => {
+  const bad = [
+    ['2026-01-15', { every: 'month' }],
+    ['2026-01-15', { every: 'day', count: 1 }],
+    ['2026-01-15', { every: 'month', count: 0 }],
+    ['2026-01-15', { every: 'month', count: 1.5 }],
+    ['2026-01-15', { every: 'month', count: 1, interval: 0 }],
+    ['2026-01-15', { every: 'month', count: 1, interval: -1 }],
+    ['2026-02-30', { every: 'month', count: 1 }],
+    ['2026-1-15', { every: 'month', count: 1 }],
+    ['2026-01-15', { every: 'month', until: '2026-13-01' }],
+    ['2027-02-29', { every: 'week', count: 1 }],
+    [undefined, { every: 'week', count: 1 }],
+  ];
+  for (const [s, r] of bad) assert.throws(() => billingDates(s, r), RangeError, JSON.stringify([s, r]));
+});