The problem here is that for flac files, RELEASETIME is contained in the Vorbis comment block that imported and hence not recognised.
I would ask over there in the JRiver forum why they don't support a formatted date that is directly human-readable for everyone (without calculation of any datediffs to 1900).
But I believe, I have found a workaround in JRiver:
JRiver let you ADD whatever field you like in
Ctrl + O (Tools -> Options) -> Library and Folders -> Manage Library Fields
There you can click on Add New Field
There you can - for example - add a new field called MY_RELEASETIME
The most important thing is: DO NOT CHOOSE Data Type Date!
Choose String as Data Type.
And activate "Save in file tags (when possible)"
After closing the Tag Panel in JRiver and reopening it, you should see your new added custom field MY_RELEASETIME between all other field names.
You can now enter whatever you want into this MY_RELEASETIME in JRiver
and this value will also be visible in Mp3tag
You can change and save MY_RELEASETIME in Mp3tag.
Then you can "Update Library (from Tags)" in JRiver and see the changed values.
This interchange works - tested - for FLAC files.
You need to test it with your FLAC files.
Please try if this also works with your existing RELEASETIME ISO date format by defining it NOT as "Date" but as "String" instead.
Just for the record:
JRiver seems to use an "internal floating-point representation... to store a date and time" and offers own internal functions like ConvertDate(...) to create such numbers.
External JRiver Source
@LyricsLover,
Many thanks for your inputs.
Your solution really works well.
I hope our exchanges and more importantly, your research will be of great use to readers of this forum who use JRiver like me.
There are a great many who have JRiver but who have historically tagged their media with MP3Tag as I have seen on the JRiver support forum.
I received a similar solution on the JRiver forum last night.
Many, Many THANKS once again.