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