Custom YYYY field

It will be awesome if the `1900 + time_info.tm_year` column can be hidden (via a customization option)
```c++
void write_date_time(char* buff, size_t buff_size)
{
auto now = system_clock::now();
long long ms_since_epoch = duration_cast(now.time_since_epoch()).count();
time_t sec_since_epoch = time_t(ms_since_epoch / 1000);
tm time_info;
localtime_r(&sec_since_epoch, &time_info);
snprintf(buff, buff_size, "%04d%02d%02d_%02d%02d%02d.%03lld",
1900 + time_info.tm_year, 1 + time_info.tm_mon, time_info.tm_mday,
time_info.tm_hour, time_info.tm_min, time_info.tm_sec, ms_since_epoch % 1000);
}
````

@jacobdufault