Support for converting Epoch to readable dates

Some tagging tools like JRiver Media Center like append a "days since Epoch" tag for full dates, instead of having a formated, readable date. JRiver stores this in JR/DATE (JR_DATE in Vorbis Comments)

JRiver then replaces the actual date field with just the year and adds it's usual shenagains onto the tags.
Before:

After. Yellow is the tags that JRiver added and red is the tags that JRiver forcefully edited. The tag enclosed in a red box is the one I want to target:

Removing the other tags is easy, but no other tagger than JRiver (and Yate, although unsure because I do not have it) is able to convert Epoch to a readable and formatted date
When the date converter is used it'll format it to either Windows' date settings, YYYY-MM-DD or a user specified format.

What exactly do you want to see?
The Tag JR_DATE with something like 2015? Or 2015-05-27?

What should 42149 be? Wednesday, May 27, 2015?
The so called "Days Since 1900-01-01"?