EDTF

[1] Extended Date/Time Format (EDTF) 1.0 Submission ( (2012-09-12 00:28:54 +09:00 版)) http://www.loc.gov/standards/datetime/pre-submission.html

[2] Extended Date Time Format (Library of Congress) ( (2012-10-31 22:06:22 +09:00 版)) http://www.loc.gov/standards/datetime/

[3] Top-level Elements and Attributes (MODS Ver. 3 User Guidelines: Metadata Object Description Schema, Library of Congress) (2016-07-22 06:48:40 +09:00) https://www.loc.gov/standards/mods/userguide/generalapp.html

edtf – This value is used for dates coded according to Extended Date/Time Format which in consistent with ISO 8601 but extends it to express special forms of dates that are not covered by w3cdtf and iso8601, such as open ended ranges.
<dateOther encoding="edtf">unknown/2009</dateOther>

[4] Extended Date Time Format (Library of Congress) (2015-09-16 22:24:55 +09:00) http://www.loc.gov/standards/datetime/

[5] Extended Date Time Format (EDTF) Specification (Library of Congress) (2019-02-05 02:55:55 +09:00) https://www.loc.gov/standards/datetime/edtf.html