Function differenceInYears

pkgs/core/src/differenceInYears/index.ts:30–66

(
  laterDate: DateArg<Date> & {},
  earlierDate: DateArg<Date> & {},
  options?: DifferenceInYearsOptions | undefined,
)

 * //=> 1
 */
export function differenceInYears(
  laterDate: DateArg<Date> & {},
  earlierDate: DateArg<Date> & {},
  options?: DifferenceInYearsOptions | undefined,
): number {
  const [laterDate_, earlierDate_] = normalizeDates(
    options?.in,
    laterDate,
    earlierDate,
  );

  // -1 if the left date is earlier than the right date
  // 2023-12-31 - 2024-01-01 = -1
  const sign = compareAsc(laterDate_, earlierDate_);

  // First calculate the difference in calendar years
  // 2024-01-01 - 2023-12-31 = 1 year
  const diff = Math.abs(differenceInCalendarYears(laterDate_, earlierDate_));

  // Now we need to calculate if the difference is full. To do that we set
  // both dates to the same year and check if the both date's month and day
  // form a full year.
  laterDate_.setFullYear(1584);
  earlierDate_.setFullYear(1584);

  // For it to be true, when the later date is indeed later than the earlier date
  // (2026-02-01 - 2023-12-10 = 3 years), the difference is full if
  // the normalized later date is also later than the normalized earlier date.
  // In our example, 1584-02-01 is earlier than 1584-12-10, so the difference
  // is partial, hence we need to subtract 1 from the difference 3 - 1 = 2.
  const partial = compareAsc(laterDate_, earlierDate_) === -sign;

  const result = sign * (diff - +partial);

  // Prevent negative zero
  return result === 0 ? 0 : result;
}