MC uses the year value as a part of the actually functional internal date stamp. In MC it isn't just a string of any numbers or other characters.
The tag value in your file is
1998 08.
MC expects to see a plain year value.
1998 would work.
Ideally MC should read the first four digits, use them if they produce a valid year value and ignore any possibly existing subsequent characters in the value string.
EDIT
As you now may have guessed, MC can tag a precise date stamp to any files in the library - to audio, video, image, etc files. It can also store the date stamps in the physical file tags if the file format supports tagging. However, in FLAC files it uses a proprietary JR_Date tag in addition to the standard tag when a more precise value than a plain year is entered. The data format of this tag is identical to what MS Excel uses internally: number of days since the reference date aka epoch (more info:
http://yabb.jriver.com/interact/index.php?topic=50750.msg346804#msg346804). This internally used value is not normally exposed in the UI. You can enter date values in the same format that is defined in the Windows locale settings.
EDIT 2
I added a link that points here to another similar problem report:
http://yabb.jriver.com/interact/index.php?topic=57470.msg395130#msg395130However, don't expect MC14 to change anymore. If JRiver changes the behavior probably the fix will appear only in MC15.
I have the latest 14 (165?)
14.0.165 is the last MC14 build. The current version is MC15.