src/ethiopic/lib/differenceInCalendarMonths.ts

import { toEthiopicDate } from "../utils/index.js";

/**
 * Difference in calendar months
 *
 * @param {Date} dateLeft - The later date
 * @param {Date} dateRight - The earlier date
 * @returns {number} The number of calendar months between the two dates
 */
export function differenceInCalendarMonths(
  dateLeft: Date,
  dateRight: Date,
): number {
  const ethiopicLeft = toEthiopicDate(dateLeft);
  const ethiopicRight = toEthiopicDate(dateRight);
  return (
    (ethiopicLeft.year - ethiopicRight.year) * 13 +
    (ethiopicLeft.month - ethiopicRight.month)
  );
}