Specifically, when in biblatex mode.
The biblatex documentation specifies that year OR date must be supplied for many types: book, article, etc.
JabRef marks entries red (erroneous) unless both year AND date are filled in; but then biber complains!
I created a patch, which fixes this issue. It also fixes the checks for e.g. @booklet, where either author OR editor must be present, and it also allows to use "journal" instead of "journaltitle" for @article (see 2.2.5 in biblatex v2.4 documentation).
This is my first patch for JabRef and I hope I got the code format correct. I followed the coding of BibtexEntryType.java, where the same kind of checks are made.