diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..fe6fa0d 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,64 @@
  * 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 ${every}`);
+  }
+  if (!Number.isInteger(interval) || interval < 1) {
+    throw new RangeError('interval must be a positive integer');
+  }
+  if (count === undefined && until === undefined) {
+    throw new RangeError('either count or until is required');
+  }
+  if (count !== undefined && (!Number.isInteger(count) || count < 1)) {
+    throw new RangeError('count must be a positive integer');
+  }
+  const [y, m, d] = parseDate(start);
+  if (until !== undefined) parseDate(until);
+  const skipped = new Set(skip);
+
+  const result = [];
+  for (let i = 0; count === undefined || result.length < count; i++) {
+    let date;
+    if (every === 'week') {
+      const dt = new Date(0);
+      dt.setUTCFullYear(y, m - 1, d + 7 * interval * i);
+      date = format(dt);
+    } else {
+      const total = m - 1 + interval * i;
+      const year = y + Math.floor(total / 12);
+      const month = total % 12;
+      date = formatParts(year, month + 1, Math.min(d, daysInMonth(year, month)));
+    }
+    if (until !== undefined && date > until) break;
+    if (!skipped.has(date)) result.push(date);
+  }
+  return result;
+}
+
+function daysInMonth(year, monthIndex) {
+  const dt = new Date(0);
+  dt.setUTCFullYear(year, monthIndex + 1, 0);
+  return dt.getUTCDate();
+}
+
+function formatParts(year, month, day) {
+  const p = (n, w) => String(n).padStart(w, '0');
+  return `${p(year, 4)}-${p(month, 2)}-${p(day, 2)}`;
+}
+
+function format(dt) {
+  return formatParts(dt.getUTCFullYear(), dt.getUTCMonth() + 1, dt.getUTCDate());
+}
+
+function parseDate(str) {
+  const match = typeof str === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(str);
+  if (!match) throw new RangeError(`invalid date: ${str}`);
+  const [y, m, d] = match.slice(1).map(Number);
+  if (m < 1 || m > 12 || d < 1 || d > daysInMonth(y, m - 1)) {
+    throw new RangeError(`invalid date: ${str}`);
+  }
+  return [y, m, d];
 }