diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..5fde74f 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,57 @@
  * 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 [y, m, d] = parseDate(start, 'start');
+  if (until !== undefined) parseDate(until, 'until');
+  const skipped = new Set(skip);
+
+  const result = [];
+  for (let k = 0; ; k++) {
+    let date;
+    if (every === 'week') {
+      date = formatDate(new Date(Date.UTC(y, m - 1, d + 7 * interval * k)));
+    } else {
+      const total = m - 1 + interval * k;
+      const year = y + Math.floor(total / 12);
+      const month = total % 12;
+      const lastDay = new Date(Date.UTC(year, month + 1, 0)).getUTCDate();
+      date = formatDate(new Date(Date.UTC(year, month, Math.min(d, lastDay))));
+    }
+    if (until !== undefined && date > until) break;
+    if (skipped.has(date)) continue;
+    result.push(date);
+    if (count !== undefined && result.length >= count) break;
+  }
+  return result;
+}
+
+function parseDate(s, name) {
+  const match = typeof s === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(s);
+  if (match) {
+    const [y, m, d] = match.slice(1).map(Number);
+    const dt = new Date(Date.UTC(y, m - 1, d));
+    dt.setUTCFullYear(y);
+    if (dt.getUTCFullYear() === y && dt.getUTCMonth() === m - 1 && dt.getUTCDate() === d) {
+      return [y, m, d];
+    }
+  }
+  throw new RangeError(`${name} must be a real calendar date in YYYY-MM-DD form`);
+}
+
+function formatDate(dt) {
+  return dt.toISOString().slice(0, 10);
 }