iOS 两日期之间相差几天

- (NSInteger)getDifferenceByDate:(NSString *)date {
        //获得当前时间
    NSDate *now = [NSDate date];
        //实例化一个NSDateFormatter对象
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
        //设定时间格式
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate *oldDate = [dateFormatter dateFromString:date];
    NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    unsigned int unitFlags = NSDayCalendarUnit;
    NSDateComponents *comps = [gregorian components:unitFlags fromDate:oldDate  toDate:now  options:0];
    return [comps day];
}