將NSString表示的日期轉換成日期格式，修正8小時偏差，計算距今的天數

- (NSInteger)convertDateString2DaysFromNowOn:(NSString *)dateStr{
    //input may like this:NSString *dateStr = @"2015-11-3 00:00:00";
    NSDateFormatter *inputFormatter = [[NSDateFormatter alloc] init] ;
    NSTimeZone* GTMzone = [NSTimeZone timeZoneForSecondsFromGMT:0];//修正日期偏差
    [inputFormatter setTimeZone:GTMzone];
    [inputFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate* inputDate = [inputFormatter dateFromString:dateStr];
    
    return (long)inputDate.timeIntervalSinceNow/86400;
}

- (BOOL)isDateStringBeyond90DaysFromNowOn:(NSString *)dateStr{
    //input may like this:NSString *dateStr = @"2015-11-3 00:00:00";
    NSDateFormatter *inputFormatter = [[NSDateFormatter alloc] init] ;
    NSTimeZone* GTMzone = [NSTimeZone timeZoneForSecondsFromGMT:0];//修正日期偏差
    [inputFormatter setTimeZone:GTMzone];
    [inputFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate* inputDate = [inputFormatter dateFromString:dateStr];
    
    NSInteger days = (long)inputDate.timeIntervalSinceNow/86400;
    return (days > 90)?YES:NO;
}

參考資料：

1.NSDateFormatter轉換時間字符串時的時區問題：http://blog.csdn.net/iunion/article/details/7242367   

2.IOS將字符串轉換爲日期時間格式：http://blog.csdn.net/gilnuy0106/article/details/8451911