Hello,
You can change it in /admin/customize/site_texts
.
Search to 'YY
will list the related date format strings.
In posts the date use this format: js.dates.tiny.date_year
Here you can change the date format 'YY
to YYYY
should be what you would like to achieve.
Note: This will change the date format everywhere it is used in Discourse, not only in posts.