By ‘member since’ do you mean the Joined section in your account summary?
If you have admin access to the server or database, you could edit the created_at parameter for that user in the Users table.
You can do it from the rails console this way:
rails c
user = User.where(username: '<your-username-here>')
user.update(created_at: "2011-02-10 00:00:00.00000")
You could also do it directly in the Postgre database (although I don’t recommend it):
update public.users set created_at='2011-02-10 00:00:00.00000' where username = '<your-username-here>';
I tested this in my local environment and worked correctly:
Changed from:
To:
Please be very careful if you decide to go with this, I suggest you take a backup of the database before doing anything 