diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..fc3f590 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,56 @@
  * 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 is required');
+  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');
+  }
+  const hasCount = count !== undefined && count !== null;
+  const hasUntil = until !== undefined && until !== null;
+  if (!hasCount && !hasUntil) throw new RangeError('count or until is required');
+  if (hasCount && (!Number.isInteger(count) || count < 1)) {
+    throw new RangeError('count must be a positive integer');
+  }
+
+  const [year, month, day] = parseDate(start, 'start');
+  if (hasUntil) 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(year, month - 1, day + 7 * interval * k)));
+    } else {
+      const m = month - 1 + interval * k;
+      const lastDay = new Date(Date.UTC(year, m + 1, 0)).getUTCDate();
+      date = formatDate(new Date(Date.UTC(year, m, Math.min(day, lastDay))));
+    }
+    if (hasUntil && date > until) break;
+    if (skipped.has(date)) continue;
+    result.push(date);
+    if (hasCount && result.length >= count) break;
+  }
+  return result;
+}
+
+function parseDate(str, name) {
+  const match = typeof str === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(str);
+  if (!match) throw new RangeError(`${name} must be a date in YYYY-MM-DD form`);
+  const [y, m, d] = [Number(match[1]), Number(match[2]), Number(match[3])];
+  const check = new Date(Date.UTC(y, m - 1, d));
+  check.setUTCFullYear(y); // years 0-99 are otherwise mapped to 1900-1999
+  if (check.getUTCFullYear() !== y || check.getUTCMonth() !== m - 1 || check.getUTCDate() !== d) {
+    throw new RangeError(`${name} is not a real calendar date`);
+  }
+  return [y, m, d];
+}
+
+function formatDate(date) {
+  return date.toISOString().slice(0, 10);
 }