import { compareAsc } from "./compareAsc.mjs"; import { differenceInCalendarMonths } from "./differenceInCalendarMonths.mjs"; import { isLastDayOfMonth } from "./isLastDayOfMonth.mjs"; import { toDate } from "./toDate.mjs"; /** * @name differenceInMonths * @category Month Helpers * @summary Get the number of full months between the given dates. * * @description * Get the number of full months between the given dates using trunc as a default rounding method. * * @typeParam DateType - The `Date` type, the function operates on. Gets inferred from passed arguments. Allows to use extensions like [`UTCDate`](https://github.com/date-fns/utc). * * @param dateLeft - The later date * @param dateRight - The earlier date * * @returns The number of full months * * @example * // How many full months are between 31 January 2014 and 1 September 2014? * const result = differenceInMonths(new Date(2014, 8, 1), new Date(2014, 0, 31)) * //=> 7 */ export function differenceInMonths(dateLeft, dateRight) { const _dateLeft = toDate(dateLeft); const _dateRight = toDate(dateRight); const sign = compareAsc(_dateLeft, _dateRight); const difference = Math.abs( differenceInCalendarMonths(_dateLeft, _dateRight), ); let result; // Check for the difference of less than month if (difference < 1) { result = 0; } else { if (_dateLeft.getMonth() === 1 && _dateLeft.getDate() > 27) { // This will check if the date is end of Feb and assign a higher end of month date // to compare it with Jan _dateLeft.setDate(30); } _dateLeft.setMonth(_dateLeft.getMonth() - sign * difference); // Math.abs(diff in full months - diff in calendar months) === 1 if last calendar month is not full // If so, result must be decreased by 1 in absolute value let isLastMonthNotFull = compareAsc(_dateLeft, _dateRight) === -sign; // Check for cases of one full calendar month if ( isLastDayOfMonth(toDate(dateLeft)) && difference === 1 && compareAsc(dateLeft, _dateRight) === 1 ) { isLastMonthNotFull = false; } result = sign * (difference - Number(isLastMonthNotFull)); } // Prevent negative zero return result === 0 ? 0 : result; } // Fallback for modularized imports: export default differenceInMonths;