下面有两种做法 

1.使用日期组件 NSDateComponents

2.使用NSString 生成一个日期

    //  创建一个日历对象

    NSCalendar *calendar = [NSCalendar currentCalendar];

    //  创建日期组件

    NSDateComponents *dc = [[NSDateComponents alloc]init];

    //  设置当前时间

    [dc setCalendar: calendar];

    [dc setYear: 2014];

    [dc setMonth: 4];

    [dc setDay: 9];

    [dc setHour:21];

    [dc setMinute:26];

    [dc setSecond:0];   

    NSString *dateStr = @"2014-04-0921:38:00";

    NSDateFormatter *formatter = [[NSDateFormatter alloc]init];

    formatter.dateFormat=@"yyyy-MM-ddHH:mm:ss";

    NSDate *schuduleNate = [dc date];

    NSDate *schuduleNate2  = [formatter dateFromString:dateStr];

    NSDate *dateNow  = [NSDate date];

    // 计算时间差

    NSTimeInterval  num = [schuduleNate2 timeIntervalSinceDate:dateNow];

    NSLog(@"%f 后开始执行任务",num);

    // 任务池

    NSRunLoop *mainLoop =   [NSRunLoop currentRunLoop];

    //

    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:num target:self selector:@selector(scheduleTask) userInfo:nil repeats:NO];

    NSTimer *timer2 = [NSTimer scheduledTimerWithTimeInterval:num+3 target:self selector:@selector(scheduleTask) userInfo:nil repeats:NO];

    // 添加到任务池中

    [mainLoop addTimer:timer forMode:NSDefaultRunLoopMode];

    [mainLoop addTimer:timer2 forMode:NSDefaultRunLoopMode];

    [mainLoop run];

输出信息

2014-04-09 21:37:54.289 WeChat[1810:60b] 5.710526后开始执行任务

2014-04-09 21:38:00.001 WeChat[1810:60b] do some thing

2014-04-09 21:38:03.001 WeChat[1810:60b] do some thing