Function differenceInDays

pkgs/core/src/differenceInDays/index.ts:63–90

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

 * //=> 92
 */
export function differenceInDays(
  laterDate: DateArg<Date> & {},
  earlierDate: DateArg<Date> & {},
  options?: DifferenceInDaysOptions | undefined,
): number {
  const [laterDate_, earlierDate_] = normalizeDates(
    options?.in,
    laterDate,
    earlierDate,
  );

  const sign = compareLocalAsc(laterDate_, earlierDate_);
  const difference = Math.abs(
    differenceInCalendarDays(laterDate_, earlierDate_),
  );

  laterDate_.setDate(laterDate_.getDate() - sign * difference);

  // Math.abs(diff in full days - diff in calendar days) === 1 if last calendar day is not full
  // If so, result must be decreased by 1 in absolute value
  const isLastDayNotFull = Number(
    compareLocalAsc(laterDate_, earlierDate_) === -sign,
  );

  const result = sign * (difference - isLastDayNotFull);
  // Prevent negative zero
  return result === 0 ? 0 : result;
}

// Like `compareAsc` but uses local time not UTC, which is needed
// for accurate equality comparisons of UTC timestamps that end up