Changing your Mastodon joined date with PostgreSQL.
Thanks to Micah F Lee for the tip! I revalidated this recently on Mastodon v4.2.8.
su mastodon
psql -d mastodon_production
mastodon_production=> UPDATE accounts SET created_at='2017-04-06 00:00:00' WHERE username='yawnbox';
exit
The change is immediate, no need to restart postgresql.