You are not the first to request filenames with a date extension. Previously I've suggested that people should log to a file and use logrotate(8) to add extensions for archival purposes (i.e. same as is done for messages and other /var/log/* files on many linux systems. I don't want to break backwards compatibility (ie if someone somewhere has "filename=$(mmdd)" and they actually *want* the file called "$(mmdd)" for some bizarre reason, I don't want it to suddenly change to "0509", but I can add a new option (e.g. "filenamex") if people really really need it.
Can you clarify what you mean by the "more precision" statement? Was that what you answered yourself below? NMEA-0183v4 timestamps can be either second precision or millisecond: kples switches with timestamp=ms or timestamp=s which of course you know, so I may be misunderstanding this (sorry).