如果app在后台待机太久，再次进来前台的时候也应该展示广告，所以在applicationDidEnterBackground的时候应该把时间存起来：

//程序切入后台,这里要注意GMT时间    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];    NSTimeZone* sourceTimeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];    [formatter setTimeZone:sourceTimeZone];    [formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];    _lastTimeEnterBackGroundStr = [formatter stringFromDate:[NSDate date]];//当前时间

在applicationWillEnterForeground的时候对比时间差，判断是否显示:

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];        NSTimeZone* sourceTimeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];        [formatter setTimeZone:sourceTimeZone];        [formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];        NSDate * lastDate = [formatter dateFromString:_lastTimeEnterBackGroundStr];        NSDate * now = [formatter dateFromString:[formatter stringFromDate:[NSDate date]]];        NSTimeInterval IntervalTime = [now timeIntervalSince1970]*1 - [lastDate timeIntervalSince1970]*1;        if (IntervalTime>(2*60*60)) {            [_mainController loadAdvertisedView];        }