diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..1e7c1a4 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -22,6 +22,69 @@
  * positive integer, if `every` isn't 'week' or 'month', or if start or until isn't a real
  * calendar date in 'YYYY-MM-DD' form.
  */
+function parseDate(value) {
+  if (typeof value !== 'string') throw new RangeError('Invalid calendar date');
+
+  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value);
+  if (!match) throw new RangeError('Invalid calendar date');
+
+  const date = new Date(`${value}T00:00:00.000Z`);
+  if (
+    date.getUTCFullYear() !== Number(match[1]) ||
+    date.getUTCMonth() !== Number(match[2]) - 1 ||
+    date.getUTCDate() !== Number(match[3])
+  ) {
+    throw new RangeError('Invalid calendar date');
+  }
+
+  return date;
+}
+
+function addMonths(start, months) {
+  const monthIndex = start.getUTCMonth() + months;
+  const year = start.getUTCFullYear() + Math.floor(monthIndex / 12);
+  const month = monthIndex % 12;
+  const date = new Date(start);
+
+  date.setUTCFullYear(year, month + 1, 0);
+  const lastDay = date.getUTCDate();
+  date.setUTCFullYear(year, month, Math.min(start.getUTCDate(), lastDay));
+  return date;
+}
+
 export function billingDates(start, rule) {
-  throw new Error('not implemented');
+  const { every, count, until, skip = [] } = rule;
+  const interval = rule.interval === undefined ? 1 : rule.interval;
+  const hasCount = count !== undefined;
+  const hasUntil = until !== undefined;
+
+  if (!hasCount && !hasUntil) throw new RangeError('count or until is required');
+  if (every !== 'week' && every !== 'month') throw new RangeError('Invalid billing frequency');
+  if (!Number.isInteger(interval) || interval <= 0) throw new RangeError('Invalid interval');
+  if (hasCount && (!Number.isInteger(count) || count <= 0)) throw new RangeError('Invalid count');
+
+  const startDate = parseDate(start);
+  const untilDate = hasUntil ? parseDate(until) : null;
+  const skipped = new Set(skip);
+  const dates = [];
+  let current = startDate;
+  let months = 0;
+
+  while (
+    (!hasCount || dates.length < count) &&
+    (!hasUntil || current <= untilDate)
+  ) {
+    const formatted = current.toISOString().slice(0, 10);
+    if (!skipped.has(formatted)) dates.push(formatted);
+
+    if (every === 'week') {
+      current = new Date(current);
+      current.setUTCDate(current.getUTCDate() + interval * 7);
+    } else {
+      months += interval;
+      current = addMonths(startDate, months);
+    }
+  }
+
+  return dates;
 }