[1] MARC 21 Format for Bibliographic Data: 008: All Materials (Network Development and MARC Standards Office, Library of Congress) (Library of Congress Network Development and MARC Standards Office著, ) https://www.loc.gov/marc/bibliographic/bd008a.html
marc – This value identifies formatted according to MARC 21 rules in field 008/07-14 for dates of publication/issuance. Thus, this would only apply to the attribute in <dateIssued>. Examples include: YYYY (for year), MMDD (for month and day), 19uu (MARC convention showing unknown digits in a year date), 9999 (MARC convention showing that the end year date has not occurred or is not known). See Legal Characters section under field 008/06 of MARC Bibliographic
<dateIssued encoding="marc">19uu</dateIssued>
[3] JAPAN/MARC MARC21フォーマットマニュアル - JAPANMARC_MARC21manual_A.pdf, , http://cldj.jp/shisho/libcat/2021/05/data/JAPANMARC_MARC21manual_A.pdf#page=39
yyyymmddhhmmss.f
yymmdd
[4] #page=52, #page=62