创建当前时间

NSDate *date = [NSDate date];  

从现在开始的24小时

        NSTimeInterval a_day = 24*60*60;

        NSDate *tomorrow = [NSDate dateWithTimeIntervalSinceNow:a_day];

根据已有日期创建日期

    NSTimeInterval a_day = 24*60*60;  
    NSDate *date = [NSDate date];  
    NSDate *tomorrow = [date addTimeInterval: date];  

日期比较

   BOOL isEqual = [date isEqualToDate:tomorrow];  
    f(isEqual){  
    NSLog(@"equal");  

//获取较早的日期

NSDate *earlier_date = [tomorrow earlierDate:date];  
  NSLog(@"earlierDate  = %@",earlierDate); 

//较晚的日期

    NSDate *laterDate = [tomorrow laterDate:date];  
    NSLog(@"laterDate  = %@",laterDate);  

//两个日期之间相隔多少秒

    NSTimeInterval secondsInterval= [date timeIntervalSinceDate:tomorrow];  
    NSLog(@"secondsInterval=  %lf",secondsInterval);  

 //通过NSCalendar类来创建日期

    NSDateComponents *comps = [[NSDateComponentsalloc]init];  
    [comps setMonth:01];  
    [comps setDay:31];  
    [comps setYear:2013];  
    NSCalendar *calendar = [[NSCalendaralloc]initWithCalendarIdentifier:NSGregorianCalendar];  
    NSDate *date = [calendar dateFromComponents:comps];  

 //从已有日期获取日期

    unsigned units  = NSMonthCalendarUnit|NSDayCalendarUnit|NSYearCalendarUnit;  
    NSDateComponents *_comps = [calendar components:units fromDate:date];  
    NSInteger month = [_comps month];  
    NSInteger year = [_comps year];  
    NSInteger day = [_comps day];  
    //NSDateFormatter实现日期的输出  
    NSDateFormatter *formatter = [[NSDateFormatteralloc]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 *dateString = [formatter stringFromDate:date];  
    NSLog(@"dateString = %@",dateString);  
    NSLog(@"formater = %@",formatter);  

//获取日期格式对象

- (NSDateFormatter *)getDateFormatter {  
    if (dateFormatter == nil) {  
    dateFormatter = [[NSDateFormatter alloc] init];  
    [dateFormatter setDateStyle:NSDateFormatterMediumStyle];  
    [dateFormatter setTimeStyle:NSDateFormatterNoStyle];  
    }  
    return dateFormatter;  
}