diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..be72728 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,61 @@
  * 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, 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 && !isPositiveInt(count)) 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 n = 0; count === undefined || result.length < count; n++) {
+    let date;
+    if (every === 'week') {
+      date = formatDate(new Date(utc(y, m - 1, d + 7 * interval * n)));
+    } else {
+      const total = m - 1 + interval * n;
+      const year = y + Math.floor(total / 12);
+      const month = total % 12;
+      const last = new Date(utc(year, month + 1, 0)).getUTCDate();
+      date = formatDate(new Date(utc(year, month, Math.min(d, last))));
+    }
+    if (until !== undefined && date > until) break;
+    if (!skipped.has(date)) result.push(date);
+  }
+  return result;
+}
+
+function isPositiveInt(v) {
+  return Number.isInteger(v) && v > 0;
+}
+
+// Date.UTC treats years 0-99 as 1900-1999, so set the year explicitly.
+function utc(year, month, day) {
+  const dt = new Date(0);
+  dt.setUTCFullYear(year, month, day);
+  return dt.getTime();
+}
+
+function parseDate(s, name) {
+  const match = typeof s === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(s);
+  if (!match) throw new RangeError(`${name} must be a 'YYYY-MM-DD' date`);
+  const [y, m, d] = match.slice(1).map(Number);
+  const dt = new Date(utc(y, m - 1, d));
+  if (dt.getUTCFullYear() !== y || dt.getUTCMonth() !== m - 1 || dt.getUTCDate() !== d) {
+    throw new RangeError(`${name} is not a real calendar date`);
+  }
+  return [y, m, d];
+}
+
+function formatDate(dt) {
+  const p = (n, w) => String(n).padStart(w, '0');
+  return `${p(dt.getUTCFullYear(), 4)}-${p(dt.getUTCMonth() + 1, 2)}-${p(dt.getUTCDate(), 2)}`;
 }