/**
 得到当前时间之后N天的日期

 @param day N天
 @return return value description
 */
- (NSDate *)getTimeAfterNowWithDay:(NSInteger)day
{
    NSDate *nowDate = [NSDate date];
    NSDate *theDate;
    
    if(day!=0)
    {
        NSTimeInterval  oneDay = 24*60*60*1;  //1天的长度
        
        theDate = [nowDate initWithTimeIntervalSinceNow: +oneDay*day ];
        //or
        //theDate = [nowDate initWithTimeIntervalSinceNow: -oneDay*day ];
    }
    else
    {
        theDate = nowDate;
    }
    return theDate;
}