Year or Date tag field is appended with 01-01 if only the year is in the source.

When reading a file, GStreamer gives only a "Date" tag.
SoundConverter does add a "year" tag since some formats needed it (can remember which ones).

Ogg Vorbis doesn't have a "year" tag normally, you can check here:
http://xiph.org/vorbis/doc/v-comment.html

Sorry for being unclear earlier. With an input .flac file that has the tag "Date=1980", the Soundconverter output .ogg or .flac will have a tag "Date=1980-01-01". Should not the output match the input?

Apetags (Wavpack, Musepack, Ape, etc...)use the "Year" tag. Just tested this and the problem remains with those formats as well. If an input .wv file has a tag "Year=1980", the Soundconverter output .ogg or .flac file will have the tag "Date=1980-01-01". I didn't have any .mpc or .ape files handy to test with.

I'm aware of the problem. But the "Date=1980-01-01" is what GStreamer pass to SoundConverter.
I'll try to see if things have changed since the last time I checked that, but don't hold your breath :)

Nothing new here. SoundConverter gets a complete date tag, without knowing which fields really were in the file.

reported upstream: https://bugzilla.gnome.org/show_bug.cgi?id=635595