diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..5f10c42 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -1,5 +1,33 @@
 // Billing schedule for subscriptions.
 
+const DAY_MS = 24 * 60 * 60 * 1000;
+const DATE_PATTERN = /^\d{4}-\d{2}-\d{2}$/;
+
+function parseDate(value) {
+  if (typeof value !== 'string' || !DATE_PATTERN.test(value)) {
+    throw new RangeError(`invalid calendar date: ${value}`);
+  }
+
+  const time = Date.parse(`${value}T00:00:00.000Z`);
+  if (!Number.isFinite(time) || new Date(time).toISOString().slice(0, 10) !== value) {
+    throw new RangeError(`invalid calendar date: ${value}`);
+  }
+  return time;
+}
+
+function addMonths(time, months) {
+  const date = new Date(time);
+  const billingDay = date.getUTCDate();
+
+  date.setUTCDate(1);
+  date.setUTCMonth(date.getUTCMonth() + months);
+
+  const lastDay = new Date(date);
+  lastDay.setUTCMonth(lastDay.getUTCMonth() + 1, 0);
+  date.setUTCDate(Math.min(billingDay, lastDay.getUTCDate()));
+  return date.getTime();
+}
+
 /**
  * Returns the dates a subscription is billed on, in order, as 'YYYY-MM-DD' strings.
  *
@@ -22,6 +50,36 @@
  * 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.
  */
-export function billingDates(start, rule) {
-  throw new Error('not implemented');
+export function billingDates(start, rule = {}) {
+  const startTime = parseDate(start);
+  const { every, interval = 1, count, until, skip = [] } = rule;
+
+  if (every !== 'week' && every !== 'month') {
+    throw new RangeError(`invalid billing frequency: ${every}`);
+  }
+  if (!Number.isInteger(interval) || interval <= 0) {
+    throw new RangeError(`invalid billing interval: ${interval}`);
+  }
+  if (count === undefined && until === undefined) {
+    throw new RangeError('count or until is required');
+  }
+  if (count !== undefined && (!Number.isInteger(count) || count <= 0)) {
+    throw new RangeError(`invalid billing count: ${count}`);
+  }
+
+  const untilTime = until === undefined ? Infinity : parseDate(until);
+  const skipped = new Set(skip);
+  const dates = [];
+
+  for (let index = 0; count === undefined || dates.length < count; index += 1) {
+    const time = every === 'week'
+      ? startTime + index * interval * 7 * DAY_MS
+      : addMonths(startTime, index * interval);
+    if (time > untilTime) break;
+
+    const date = new Date(time).toISOString().slice(0, 10);
+    if (!skipped.has(date)) dates.push(date);
+  }
+
+  return dates;
 }