It would be very helpful if admins were allowed to change the "joined" date of members, since sometimes new logins are created and don't reflect how long a member (or member turned moderator!) has been present in a forum. 

Thank you!

There’s a 3rd-party resource for this:

 

https://www.sosinvision.com.br/index.php?/file/128-change-member-joined-date/

@Adriano Faria oh thank you! Do you know if this application has to stay installed for the date joined change to remain in place? Or is it a permanent change, once made? 

Permanent. Once changed, it’s done. You can uninstall the app or leave it there to change date from this or other members.