用nstimer实现倒计时功能，废话不多说，直接上代码，详细解释请参照注释
//
[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFireMethod:) userInfo:nil repeats:YES];
//
- (void)timerFireMethod:(NSTimer *)theTimer
{
BOOL timeStart = YES;
NSCalendar *cal = [NSCalendar currentCalendar];//定义一个NSCalendar对象
NSDateComponents *endTime = [[NSDateComponents alloc] init]; //初始化目标时间...
NSDate *today = [NSDate date]; //得到当前时间
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSDate *dateString = [dateFormatter dateFromString:todate];
NSString *overdate = [dateFormatter stringFromDate:dateString];
// NSLog(@"overdate=%@",overdate);
static int year;
static int month;
static int day;
static int hour;
static int minute;
static int second;
if(timeStart) {//从NSDate中取出年月日，时分秒，但是只能取一次
year = [[overdate substringWithRange:NSMakeRange(0, 4)] intValue];
month = [[overdate substringWithRange:NSMakeRange(5, 2)] intValue];
day = [[overdate substringWithRange:NSMakeRange(8, 2)] intValue];
hour = [[overdate substringWithRange:NSMakeRange(11, 2)] intValue];
minute = [[overdate substringWithRange:NSMakeRange(14, 2)] intValue];
second = [[overdate substringWithRange:NSMakeRange(17, 2)] intValue];
timeStart= NO;
}
[endTime setYear:year];
[endTime setMonth:month];
[endTime setDay:day];
[endTime setHour:hour];
[endTime setMinute:minute];
[endTime setSecond:second];
NSDate *overTime = [cal dateFromComponents:endTime]; //把目标时间装载入date
//用来得到具体的时差，是为了统一成北京时间
unsigned int unitFlags = NSYearCalendarUnit| NSMonthCalendarUnit| NSDayCalendarUnit| NSHourCalendarUnit| NSMinuteCalendarUnit| NSSecondCalendarUnit;
NSDateComponents *d = [cal components:unitFlags fromDate:today toDate:overTime options:0];
NSString *t = [NSString stringWithFormat:@"%d", [d day]];
NSString *h = [NSString stringWithFormat:@"%d", [d hour]];
NSString *fen = [NSString stringWithFormat:@"%d", [d minute]];
if([d minute] < 10) {
fen = [NSString stringWithFormat:@"0%d",[d minute]];
}
NSString *miao = [NSString stringWithFormat:@"%d", [d second]];
if([d second] < 10) {
miao = [NSString stringWithFormat:@"0%d",[d second]];
}
// NSLog(@"===%@天 %@:%@:%@",t,h,fen,miao);
[_longtime setText:[NSString stringWithFormat:@"%@天 %@:%@:%@",t,h,fen,miao]];
if([d second] > 0) {
//计时尚未结束，do_something
// [_longtime setText:[NSString stringWithFormat:@"%@:%@:%@",d,fen,miao]];
} else if([d second] == 0) {
//计时结束 do_something
} else{
//计时器失效
[theTimer invalidate];
}
}