diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..022d1e8 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,74 @@
  * calendar date in 'YYYY-MM-DD' form.
  */
 export function billingDates(start, rule) {
-  throw new Error('not implemented');
+  if (rule.every !== 'week' && rule.every !== 'month') {
+    throw new RangeError("every must be 'week' or 'month'");
+  }
+
+  const interval = rule.interval === undefined ? 1 : rule.interval;
+  if (!Number.isInteger(interval) || interval <= 0) {
+    throw new RangeError('interval must be a positive integer');
+  }
+
+  const hasCount = rule.count !== undefined;
+  if (!hasCount && rule.until === undefined) {
+    throw new RangeError('count or until is required');
+  }
+  if (hasCount && (!Number.isInteger(rule.count) || rule.count <= 0)) {
+    throw new RangeError('count must be a positive integer');
+  }
+
+  const first = parseDate(start);
+  const last = rule.until === undefined ? undefined : parseDate(rule.until);
+  const skip = new Set(rule.skip ?? []);
+  const dates = [];
+  let occurrence = 0;
+
+  while (!hasCount || dates.length < rule.count) {
+    const date = rule.every === 'week'
+      ? addDays(first, occurrence * interval * 7)
+      : addMonths(first, occurrence * interval);
+
+    if (last !== undefined && date > last) break;
+
+    const formatted = formatDate(date);
+    if (!skip.has(formatted)) dates.push(formatted);
+    occurrence += 1;
+  }
+
+  return dates;
+}
+
+function parseDate(value) {
+  if (typeof value !== 'string' || !/^\d{4}-\d{2}-\d{2}$/.test(value)) {
+    throw new RangeError('date must be a real calendar date in YYYY-MM-DD form');
+  }
+
+  const date = new Date(`${value}T00:00:00.000Z`);
+  if (Number.isNaN(date.getTime()) || formatDate(date) !== value) {
+    throw new RangeError('date must be a real calendar date in YYYY-MM-DD form');
+  }
+  return date;
+}
+
+function formatDate(date) {
+  return date.toISOString().slice(0, 10);
+}
+
+function addDays(date, days) {
+  const result = new Date(date);
+  result.setUTCDate(result.getUTCDate() + days);
+  return result;
+}
+
+function addMonths(date, months) {
+  const day = date.getUTCDate();
+  const result = new Date(date);
+  result.setUTCDate(1);
+  result.setUTCMonth(result.getUTCMonth() + months);
+
+  const lastDay = new Date(result);
+  lastDay.setUTCMonth(lastDay.getUTCMonth() + 1, 0);
+  result.setUTCDate(Math.min(day, lastDay.getUTCDate()));
+  return result;
 }