diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..1f56b76 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,62 @@
  * 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;
+
+  if (count === undefined && until === undefined) {
+    throw new RangeError('count or until is required');
+  }
+  if (every !== 'week' && every !== 'month') {
+    throw new RangeError("every must be 'week' or 'month'");
+  }
+  if (!Number.isInteger(interval) || interval <= 0) {
+    throw new RangeError('interval must be a positive integer');
+  }
+  if (count !== undefined && (!Number.isInteger(count) || count <= 0)) {
+    throw new RangeError('count must be a positive integer');
+  }
+
+  const startDate = parseDate(start);
+  const untilDate = until === undefined ? undefined : parseDate(until);
+  const skipped = new Set(skip);
+  const dates = [];
+
+  for (let occurrence = 0; count === undefined || dates.length < count; occurrence++) {
+    const date = new Date(startDate);
+
+    if (every === 'week') {
+      date.setUTCDate(date.getUTCDate() + occurrence * interval * 7);
+    } else {
+      // Always advance from the original month so shortened months don't move the anchor.
+      date.setUTCDate(1);
+      date.setUTCMonth(date.getUTCMonth() + occurrence * interval);
+      const lastDay = new Date(date);
+      lastDay.setUTCMonth(lastDay.getUTCMonth() + 1, 0);
+      date.setUTCDate(Math.min(startDate.getUTCDate(), lastDay.getUTCDate()));
+    }
+
+    if (untilDate !== undefined && date > untilDate) break;
+
+    const formatted = date.toISOString().slice(0, 10);
+    if (!skipped.has(formatted)) dates.push(formatted);
+  }
+
+  return dates;
+}
+
+function parseDate(value) {
+  const match = typeof value === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(value);
+  if (!match) throw new RangeError('date must be a real calendar date in YYYY-MM-DD form');
+
+  const year = Number(match[1]);
+  const month = Number(match[2]);
+  const day = Number(match[3]);
+  const date = new Date(0);
+  date.setUTCFullYear(year, month - 1, day);
+
+  if (date.getUTCFullYear() !== year || date.getUTCMonth() !== month - 1 || date.getUTCDate() !== day) {
+    throw new RangeError('date must be a real calendar date in YYYY-MM-DD form');
+  }
+
+  return date;
 }