diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..405e22b 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,66 @@
  * 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 must be an object');
+  }
+  const { every, until, skip = [] } = rule;
+  const interval = rule.interval === undefined ? 1 : rule.interval;
+  const count = rule.count === null ? undefined : rule.count;
+  const hasUntil = until !== undefined && until !== null;
+
+  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 && !hasUntil) {
+    throw new RangeError('either count or until is required');
+  }
+  const [y, m, d] = parseDate(start, 'start');
+  const untilStr = hasUntil ? formatDate(...parseDate(until, 'until')) : null;
+  const skipped = new Set(skip);
+
+  const result = [];
+  for (let i = 0; count === undefined || result.length < count; i++) {
+    let date;
+    if (every === 'week') {
+      const t = new Date(0);
+      t.setUTCFullYear(y, m - 1, d + 7 * interval * i);
+      date = formatDate(t.getUTCFullYear(), t.getUTCMonth() + 1, t.getUTCDate());
+    } else {
+      const total = y * 12 + (m - 1) + interval * i;
+      const year = Math.floor(total / 12);
+      const month = (total % 12) + 1;
+      date = formatDate(year, month, Math.min(d, daysInMonth(year, month)));
+    }
+    if (untilStr !== null && date > untilStr) break;
+    if (!skipped.has(date)) result.push(date);
+  }
+  return result;
+}
+
+function isPositiveInt(n) {
+  return Number.isInteger(n) && n > 0;
+}
+
+function daysInMonth(year, month) {
+  const leap = (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
+  return month === 2 ? (leap ? 29 : 28) : [4, 6, 9, 11].includes(month) ? 30 : 31;
+}
+
+function parseDate(s, name) {
+  const match = typeof s === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(s);
+  if (match) {
+    const [y, m, d] = match.slice(1).map(Number);
+    if (m >= 1 && m <= 12 && d >= 1 && d <= daysInMonth(y, m)) return [y, m, d];
+  }
+  throw new RangeError(`${name} must be a real date in YYYY-MM-DD form`);
+}
+
+function formatDate(y, m, d) {
+  const p = (n, w) => String(n).padStart(w, '0');
+  return `${p(y, 4)}-${p(m, 2)}-${p(d, 2)}`;
 }