diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..e00c80b 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,70 @@
  * 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, until, skip = [] } = rule;
+  const interval = rule.interval === undefined ? 1 : rule.interval;
+  const count = rule.count;
+
+  if (every !== 'week' && every !== 'month') throw new RangeError("every must be 'week' or 'month'");
+  if (!isPositiveInt(interval)) throw new RangeError('interval must be a positive integer');
+  if (count === undefined && until === undefined) throw new RangeError('count or until is required');
+  if (count !== undefined && !isPositiveInt(count)) throw new RangeError('count must be a positive integer');
+  if (!Array.isArray(skip)) throw new RangeError('skip must be an array');
+
+  const first = parseDate(start, 'start');
+  if (until !== undefined) parseDate(until, 'until');
+
+  const skipped = new Set(skip);
+  const dates = [];
+  // Each date is computed from the start date, not from the previous date, so a clamped
+  // day (Feb 28) doesn't carry over into later months.
+  for (let i = 0; count === undefined || dates.length < count; i++) {
+    const date = every === 'week'
+      ? addDays(first, 7 * interval * i)
+      : addMonths(first, interval * i);
+    if (date === null) break; // ran past year 9999
+    if (until !== undefined && date > until) break;
+    if (!skipped.has(date)) dates.push(date);
+  }
+  return dates;
+}
+
+function isPositiveInt(n) {
+  return Number.isInteger(n) && n > 0;
+}
+
+function daysInMonth(year, month) {
+  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`);
+}
+
+function format({ year, month, day }) {
+  if (year > 9999) return null;
+  const p = (n, w) => String(n).padStart(w, '0');
+  return `${p(year, 4)}-${p(month, 2)}-${p(day, 2)}`;
+}
+
+function addMonths({ year, month, day }, n) {
+  const index = year * 12 + (month - 1) + n;
+  const y = Math.floor(index / 12);
+  const mo = (index % 12) + 1;
+  return format({ year: y, month: mo, day: Math.min(day, daysInMonth(y, mo)) });
+}
+
+function addDays({ year, month, day }, n) {
+  const d = new Date(0);
+  d.setUTCFullYear(year, month - 1, day + n);
+  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..1bd220f 100644
--- a/tests/schedule.test.js
+++ b/tests/schedule.test.js
@@ -2,6 +2,39 @@ import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { billingDates } from '../src/schedule.js';
 
+test('month-end clamps then 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('2024-02-29', { every: 'month', interval: 12, count: 3 }), ['2024-02-29', '2025-02-28', '2026-02-28']);
+});
+
+test('weekly with until and skip across a year boundary', () => {
+  assert.deepEqual(
+    billingDates('2026-12-30', { every: 'week', until: '2027-01-20', skip: ['2027-01-06'] }),
+    ['2026-12-30', '2027-01-13', '2027-01-20'],
+  );
+});
+
+test('skipped dates do not count toward count', () => {
+  assert.deepEqual(billingDates('2026-01-01', { every: 'week', count: 2, skip: ['2026-01-08'] }), ['2026-01-01', '2026-01-15']);
+});
+
+test('count and until stop at whichever comes first', () => {
+  assert.deepEqual(billingDates('2026-05-01', { every: 'month', count: 3, until: '2026-05-31' }), ['2026-05-01']);
+  assert.deepEqual(billingDates('2026-06-01', { every: 'month', count: 3, until: '2026-05-31' }), []);
+});
+
+test('invalid input throws RangeError', () => {
+  const bad = [
+    ['2026-02-30', { every: 'week', count: 1 }],
+    ['2026-01-01', { every: 'week' }],
+    ['2026-01-01', { every: 'day', count: 1 }],
+    ['2026-01-01', { every: 'week', count: 0 }],
+    ['2026-01-01', { every: 'week', interval: 1.5, count: 1 }],
+    ['2026-01-01', { every: 'week', until: '2026-13-01' }],
+  ];
+  for (const [s, r] of bad) assert.throws(() => billingDates(s, r), RangeError);
+});
+
 test('monthly, mid-month start', () => {
   assert.deepEqual(billingDates('2026-01-15', { every: 'month', count: 3 }), ['2026-01-15', '2026-02-15', '2026-03-15']);
 });