diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..d53e870 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 ?? {};
+
+  const parse = (s, name) => {
+    const m = typeof s === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(s);
+    if (!m) throw new RangeError(`${name} must be a 'YYYY-MM-DD' date`);
+    const [y, mo, d] = [Number(m[1]), Number(m[2]), Number(m[3])];
+    if (mo < 1 || mo > 12 || d < 1 || d > daysInMonth(y, mo - 1)) {
+      throw new RangeError(`${name} is not a real calendar date`);
+    }
+    return { y, mo: mo - 1, d };
+  };
+  const pad = (n, w) => String(n).padStart(w, '0');
+  const format = (y, mo, d) => `${pad(y, 4)}-${pad(mo + 1, 2)}-${pad(d, 2)}`;
+  const isPositiveInt = (n) => Number.isInteger(n) && n > 0;
+
+  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');
+  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 && !isPositiveInt(count)) throw new RangeError('count must be a positive integer');
+
+  const first = parse(start, 'start');
+  if (hasUntil) parse(until, 'until');
+  const skipped = new Set(skip);
+
+  const result = [];
+  for (let k = 0; !hasCount || result.length < count; k++) {
+    let y, mo, d;
+    if (every === 'week') {
+      const t = new Date(0);
+      t.setUTCFullYear(first.y, first.mo, first.d + 7 * interval * k);
+      [y, mo, d] = [t.getUTCFullYear(), t.getUTCMonth(), t.getUTCDate()];
+    } else {
+      const total = first.mo + interval * k;
+      y = first.y + Math.floor(total / 12);
+      mo = total % 12;
+      d = Math.min(first.d, daysInMonth(y, mo));
+    }
+    if (y > 9999) break;
+    const date = format(y, mo, d);
+    if (hasUntil && date > until) break;
+    if (!skipped.has(date)) result.push(date);
+  }
+  return result;
+}
+
+function daysInMonth(y, mo) {
+  const t = new Date(0);
+  t.setUTCFullYear(y, mo + 1, 0);
+  return t.getUTCDate();
 }