Incorrert handling of DATE metadata in FLAC

And other Vorbis metadata containers
mp3tag threats DATE metadata as YEAR, which is not used in Vorbis metadata format. When you write DATA manually mp3tag can save correctly, but on next opening it anyway shows YEAR
VorbisComment - XiphWiki

Please see Tools>Options>Mapping that YEAR and DATE get a special treatment so that the mp3 field YEAR becomes DATE in flac files.

Wow, it's so unclear..

I assume that you want to enter data in a GUI element that is somehow labelled "Year" to save a time coordinate.
And I also assume that this should be possible ireespective of the file type.
So enter the year in YEAR and MP3tag will take care of it so that it gets saved in the correct field/atom/chunk/vorbis comment.
The alternative would be that you have (at least) 2 input areas: one for date and one for year - and you would have to watch out that you fill the correct one.

Your observation that you see the data from "date" in YEAR next time you open MP3tag proves that the mapping works.
Does this make things clearer? Or where do you have further problems?