diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..b9a1842 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,60 @@
  * 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'");
+  }
+  if (!Number.isInteger(interval) || interval < 1) {
+    throw new RangeError('interval must be a positive integer');
+  }
+  if (count !== undefined && (!Number.isInteger(count) || count < 1)) {
+    throw new RangeError('count must be a positive integer');
+  }
+  if (count === undefined && until === undefined) {
+    throw new RangeError('count or until is required');
+  }
+  const startParts = parseDate(start, 'start');
+  if (until !== undefined) parseDate(until, 'until');
+  const skipped = new Set(skip);
+
+  const [y, m, d] = startParts;
+  const result = [];
+  for (let i = 0; count === undefined || result.length < count; i++) {
+    let date;
+    if (every === 'week') {
+      const t = new Date(0);
+      t.setUTCFullYear(y, m - 1, d + 7 * interval * i);
+      date = format(t.getUTCFullYear(), t.getUTCMonth() + 1, t.getUTCDate());
+    } else {
+      const total = y * 12 + (m - 1) + interval * i;
+      const year = Math.floor(total / 12);
+      const month = (total % 12) + 1;
+      date = format(year, month, Math.min(d, daysInMonth(year, month)));
+    }
+    // 'YYYY-MM-DD' strings compare chronologically
+    if (until !== undefined && date > until) break;
+    if (!skipped.has(date)) result.push(date);
+  }
+  return result;
+}
+
+function daysInMonth(year, month) {
+  const t = new Date(0);
+  t.setUTCFullYear(year, month, 0);
+  return t.getUTCDate();
+}
+
+function format(y, m, d) {
+  return `${String(y).padStart(4, '0')}-${String(m).padStart(2, '0')}-${String(d).padStart(2, '0')}`;
+}
+
+function parseDate(s, name) {
+  const match = typeof s === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(s);
+  if (match) {
+    const [y, m, d] = match.slice(1).map(Number);
+    if (m >= 1 && m <= 12 && d >= 1 && d <= daysInMonth(y, m)) return [y, m, d];
+  }
+  throw new RangeError(`${name} must be a real calendar date in YYYY-MM-DD form`);
 }