mpeg date not hyphenated

[532910]

Checklist

• I'm reporting a broken site support issue
• [ x] I've verified that I'm running youtube-dl version 2020.09.06
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• [x ] I've searched the bugtracker for similar bug reports including closed ones
• [x ] I've read bugs section in FAQ

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Description

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While --xattrs produces hyphenated data

youtube-dl/youtube_dl/postprocessor/xattrpp.py

Line 52 in 3089bc7

value = hyphenate_date(value)

--add-metadata does not:

% getfattr -dm- file.mp3 | grep date
user.dublincore.date="2015-05-29"
% mediainfo file.mp3 | grep date
Recorded date                            : 20150529

[532910]

What's wrong to store date in the iso format everywhere, even json meta?