diff --git a/src/schedule.js b/src/schedule.js
index 862dc55..e4476be 100644
--- a/src/schedule.js
+++ b/src/schedule.js
@@ -23,5 +23,80 @@
  * 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 (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 && until === undefined) {
+    throw new RangeError('count or until is required');
+  }
+  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 = [];
+  let date = startDate;
+  let monthsElapsed = 0;
+
+  while (
+    (count === undefined || dates.length < count) &&
+    (untilDate === undefined || date <= untilDate)
+  ) {
+    const formatted = formatDate(date);
+    if (!skipped.has(formatted)) dates.push(formatted);
+
+    if (count !== undefined && dates.length === count) break;
+
+    if (every === 'week') {
+      date = calendarDate(
+        date.getUTCFullYear(),
+        date.getUTCMonth(),
+        date.getUTCDate() + interval * 7,
+      );
+    } else {
+      monthsElapsed += interval;
+      const monthIndex = startDate.getUTCFullYear() * 12 + startDate.getUTCMonth() + monthsElapsed;
+      const year = Math.floor(monthIndex / 12);
+      const month = monthIndex % 12;
+      const lastDay = calendarDate(year, month + 1, 0).getUTCDate();
+      date = calendarDate(year, month, Math.min(startDate.getUTCDate(), lastDay));
+    }
+  }
+
+  return dates;
+}
+
+function parseDate(value) {
+  const match = typeof value === 'string' && /^(\d{4})-(\d{2})-(\d{2})$/.exec(value);
+  if (!match) throw new RangeError('date must be a real calendar date in YYYY-MM-DD form');
+
+  const year = Number(match[1]);
+  const month = Number(match[2]) - 1;
+  const day = Number(match[3]);
+  const date = calendarDate(year, month, day);
+
+  if (date.getUTCFullYear() !== year || date.getUTCMonth() !== month || date.getUTCDate() !== day) {
+    throw new RangeError('date must be a real calendar date in YYYY-MM-DD form');
+  }
+  return date;
+}
+
+function calendarDate(year, month, day) {
+  const date = new Date(0);
+  date.setUTCFullYear(year, month, day);
+  return date;
+}
+
+function formatDate(date) {
+  const year = String(date.getUTCFullYear()).padStart(4, '0');
+  const month = String(date.getUTCMonth() + 1).padStart(2, '0');
+  const day = String(date.getUTCDate()).padStart(2, '0');
+  return `${year}-${month}-${day}`;
 }