Go to: http://www.4webhelp.net/us/timestamp.php
Fill in your correct registration date in the second section.
Click "Convert to a TimeStamp"
A gray box appears above it.
Take the long number it generates, and copy it.
Go into your database. (phpMyAdmin)
Find the phpbb_users table.
Click the Browse tab.
Find your username.
Find your registration field and paste in the new number.
It should be similar to that. I'm just doing that from memory.