时间处理NSDate

    NSTimeInterval now = [NSDate timeIntervalSinceReferenceDate];  //2001/1/1时间以秒数的形式表示
    NSLog(@"double=%f",now);//444907472.111701
    //
    NSDate *datec = [NSDate dateWithTimeIntervalSinceNow:100];
    NSTimeInterval seconds = [datec timeIntervalSinceNow]; //日期于当前日期的相差时间
    NSLog(@"%f",seconds);//以秒数的形式
    //
    NSDate *datea = [NSDate dateWithTimeIntervalSince1970:0];
    NSDate *dateb = [NSDate dateWithTimeInterval:50 sinceDate:datea];
    NSTimeInterval secondsa = [datea timeIntervalSinceDate:dateb];//日期与日期的相差时间
    NSLog(@"%f",secondsa);
    /////////////////////////////////////////////////////NSDate的创建/////////////////////////////////////////////////////////////////////////////
    NSDate *date = [NSDate date];
    NSLog(@"date=%@",date);
    //以当前时间为基准
    NSDate *timeInterval = [[NSDate alloc]initWithTimeIntervalSinceNow:20];
    NSLog(@"timeInterval= %@",timeInterval);
    //以1970/1/1为基准
    NSDate *since1970= [[NSDate alloc]initWithTimeIntervalSince1970:-20];
    NSLog(@"since1970=%@",since1970);
    //以2001/1/1为基准
    NSDate *sinceReferenceDate = [[NSDate alloc]initWithTimeIntervalSinceReferenceDate:80];
    NSLog(@"sinceReferenceDate=%@",sinceReferenceDate);
    //以某日期为基准
    NSDate *date2 = [[NSDate alloc]initWithTimeInterval:10 sinceDate:date];
    NSLog(@"date2=%@",date2);
    //
    NSLog(@"future date = %@",[NSDate distantFuture]);
    NSLog(@"past date = %@",[NSDate distantPast]);

2015-02-06 17:49:38.290 MeiTeamDemo[13580:203531] double=444908978.290587
2015-02-06 17:49:38.291 MeiTeamDemo[13580:203531] 99.999974
2015-02-06 17:49:38.291 MeiTeamDemo[13580:203531] -50.000000
2015-02-06 17:49:38.292 MeiTeamDemo[13580:203531] date=2015-02-06 09:49:38 +0000
2015-02-06 17:49:38.293 MeiTeamDemo[13580:203531] timeInterval= 2015-02-06 09:49:58 +0000
2015-02-06 17:49:38.293 MeiTeamDemo[13580:203531] since1970=1969-12-31 23:59:40 +0000
2015-02-06 17:49:38.293 MeiTeamDemo[13580:203531] sinceReferenceDate=2001-01-01 00:01:20 +0000
2015-02-06 17:49:38.293 MeiTeamDemo[13580:203531] date2=2015-02-06 09:49:48 +0000
2015-02-06 17:49:38.293 MeiTeamDemo[13580:203531] future date = 4001-01-01 00:00:00 +0000
2015-02-06 17:49:38.293 MeiTeamDemo[13580:203531] past date = 0000-12-30 00:00:00 +0000

CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();  
// do something  
 CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();  
 NSLog(@"time cost: %0.3f", end - start);  

NSTimeInterval interval = [_startDate timeIntervalSinceNow];//日期相差的秒数
    
    NSString *title = [NSString stringWithFormat:@"做题进度：%ld/%ld 道  剩余时间：%d:%d",_indexOfQuestion + 1, self.dataSource.count ,(int)interval/60,(int)interval%60 ];