I have been trying to encode from some lossless format to .mp3 using Foobar2k's encoder. This is my command line:

--add-id3v2 -V 1 - %d

This works for converting the file.

However, I have also tried:

--add-id3v2 -ty "%date%" -V 1 - %d
--add-id3v2 -ty %date% -V 1 - %d
--add-id3v2 -ty "%DATE%" -V 1 - %d
and
--add-id3v2 -ty %DATE% -V 1 - %d

with no luck. Does anyone have some info on tagging .mp3-files in F2K when encoding?

A) There's a foobar2000 forum for such questions
B) You don't need to specify tags via commandline. Foobar2000 will copy existing tags over to the new file automatically. If transcoding to mp3 you have to configure the tag type you want under "standard inputs".