JFTR and judging from the Exiftool screenshot only, Date/Time Original in UserData is apparently the date metadata box/atom, containing a string. The other dates are stored as numbers in the respective headers.
https://www.exiftool.org/TagNames/QuickTime.html#UserData
I’m not sure why Apple isn’t using ©day (“Content Create Date”) here, but it’s not unreasonable to ask for (read-only?) support for date in Mp3tag.