Oc Base64 引起的崩溃，日期格式问题

NSData *data = [[NSData alloc] initWithBase64EncodedString:@"TU1ZWVlZREQ=" options:NSDataBase64DecodingIgnoreUnknownCharacters];
NSString *string = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:string];

//
NSData *data1 = [[NSData alloc] initWithBase64EncodedString: @"MDEyMDIwMTA=" options:NSDataBase64DecodingIgnoreUnknownCharacters];
NSString *string1 = [[NSString alloc] initWithData:data1 encoding:NSUTF8StringEncoding];
NSDate *dst_d = [formatter dateFromString:string1];

NSDate *cur_d = [NSDate date];
NSInteger ret = [cur_d timeIntervalSinceDate:dst_d];
NSLog(@"%ld", (long)ret);
if (ret > 0){
    return true;
}

在iphone 6s上出现概率性闪退
错误日志显示
<Notice>: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[_NSPlaceholderData initWithBase64EncodedString:options:]: nil string argument'
*** First throw call stack:

常量字符串为啥会为nil呢？ 很费解，求解释

去掉base64之后 发现

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"DDYYYYMM"];
NSDate *dst_d = [formatter dateFromString:@"14202001"];

得出来的结果。dst_d 的值是。
dst_d __NSTaggedDate * 2019-12-21 16:00:00 UTC 0xe41c1d73c8000000

不明白。非常之诡异
改成这样？

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"MMYYYYDD"];
NSDate *dst_d = [formatter dateFromString:@"01202014"];

正常了！！！

NSDate *cur_d = [NSDate date];
NSInteger ret = [cur_d timeIntervalSinceDate:dst_d];
NSLog(@"%ld", (long)ret);
if (ret > 0){
    return true;
}