// 当前时间创建NSDate
        
        NSDate *myDate = [NSDate date];
        
        NSLog(@"myDate = %@",myDate);
        
        //从现在开始的24小时
        
        NSTimeInterval secondsPerDay = 24*60*60;
        
        NSDate *tomorrow = [NSDate dateWithTimeIntervalSinceNow:secondsPerDay];
        
        NSLog(@"tomorrow = %@",tomorrow);
        
        //根据已有日期创建日期
        
        NSTimeInterval secondsPerDay1 = 24*60*60;
        
        NSDate *now = [NSDate date];
        
        NSDate *yesterDay = [now dateByAddingTimeInterval:-secondsPerDay1];
        
        NSLog(@"yesterDay = %@",yesterDay);
        
        //比较日期
        
        BOOL sameDate = [now isEqualToDate:yesterDay];
        
        NSLog(@"sameDate = %hhd",sameDate);
        
        //获取较早的日期
        
        NSDate *earlierDate = [yesterDay earlierDate:now];
        
        NSLog(@"earlierDate  = %@",earlierDate);
        
        //获取较晚的日期
        
        NSDate *laterDate = [yesterDay laterDate:now];
        
        NSLog(@"laterDate  = %@",laterDate);
        
        //两个日期之间相隔多少秒
        
        NSTimeInterval secondsBetweenDates= [yesterDay timeIntervalSinceDate:now];
        
        NSLog(@"secondsBetweenDates=  %lf",secondsBetweenDates);
        
        //通过NSCalendar类来创建日期
        
        NSDateComponents *comp = [[NSDateComponents alloc]init];
        
        [comp setMonth:06];
        
        [comp setDay:12];
        
        [comp setYear:2015];
        
        NSCalendar *myCal= [[NSCalendar alloc]initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
        
        NSDate *myDate1 = [myCal dateFromComponents:comp];
        
        NSLog(@"myDate1 = %@",myDate1);
        
        //从已有日期获取日期
        
        unsigned units  = NSCalendarUnitMonth|NSCalendarUnitDay|NSCalendarUnitYear;
        
        NSDateComponents *comp1 = [myCal components:units fromDate:now];
        
        NSInteger month = [comp1 month];
        
        NSInteger year = [comp1 year];
        
        NSInteger day = [comp1 day];
        
        //NSDateFormatter实现日期的输出
        
        NSDateFormatter *formatter = [[NSDateFormatter alloc]init];
        
        [formatter setDateStyle:NSDateFormatterFullStyle];//直接输出的话是机器码
        
        //或者是手动设置样式[formatter setDateFormat:@"yyyy-mm-dd"];
        
        NSString *string = [formatter stringFromDate:now];
        
        NSLog(@"string = %@",string);
        
        NSLog(@"formater = %@",formatter);
        
        //用NSCalendar实现，计算某个时间C是某个日期B是相应时期A，中的第几(Hour?Day?Month?)
        
        NSCalendar *cal = [NSCalendar currentCalendar];
        
        NSLog(@"My calendar is %@",[cal calendarIdentifier]);//获取日期格式对象
        
        unsigned long daymonth = [cal ordinalityOfUnit:NSCalendarUnitDay//时间B(天)
                             
                                           inUnit:NSCalendarUnitMonth//时间A(月)
                             
                                          forDate:now];//时间C(当前时间)
        
        NSLog(@"This is day %lu of the month",daymonth);//当前时间C，是一个月(A)的第几天(B)