Find the number of overlapping days of 4 NSDates

NSDate+Extensions.m

- (BOOL) isBetweenBeginDate:(NSDate *)beginDate endDate:(NSDate *)endDate
{
    return ([self compare:beginDate] == NSOrderedDescending || [self compare:beginDate] == NSOrderedSame) && ([self compare:endDate] == NSOrderedAscending);
}

- (NSInteger)distanceInDaysToDate:(NSDate *)anotherDate
{
    NSCalendar *gregorianCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDateComponents *components = [gregorianCalendar components:NSDayCalendarUnit fromDate:self toDate:anotherDate options:0];
    return components.day;
}

+ (NSInteger) countOverlappingDaysOfFirstStartDate:(NSDate *)firstStartDate firstEndDate:(NSDate *)firstEndDate secondStartDate:(NSDate *)secondStartDate secondEndDate:(NSDate *)secondEndDate
{
    NSArray *dates = @[firstStartDate,firstEndDate,secondStartDate,secondEndDate];
    NSArray *sortedDates = [dates sortedArrayUsingComparator:^NSComparisonResult(NSDate *a, NSDate *b)
    {
        return [a compare:b];
    }];
    if (![firstStartDate isBetweenBeginDate:secondStartDate endDate:secondEndDate] && ![firstEndDate isBetweenBeginDate:secondStartDate endDate:secondEndDate]) return 0;
    return [sortedDates[1] distanceInDaysToDate:sortedDates[2]];
}