Currently, the Year Only doesn't provide useful default value options. The only settings are '- None -' and '0'.
Default values could provide an improved user experience, e.g in cases where there is a large range of years, or the current year is most-commonly used.
Implement YearOnlyFieldItemList to provide useful default options (e.g. current year, relative year, specific year).
Add a select for default value type. Provide dependent fields to collect relative date string (strtotime) and/or specific year value.
Add default_type, default_relative and default_specific to schema.
Needs review
9.1
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Doesn't work on Drupal 10.3
Unfortunately I'm not a developer, can someone fix it, please?
Here's a patch re-rolled for 9.1.1 and/or 9.x-dev. Please give it a try.
Patch #14 worked for me with the latest 9.1.2 version of the module.