Right this might be a bit complicated. if you know how to manualy change your database you'll be ok.
Back Up Your DataBase before making any changes.
i take it that in your admin panel the start date is also wrong. so we will start with that. Copy and paste this into notepad.
PHP Code:
update phpbb_config
set config_value = unix_timestamp('2007-02-02 12:00:00')
where config_name = 'board_startdate'
Change the date to the date you started your forum, save as sql into a place you can find it. Now go to your Cpanel and click mysql when the page opens scroll right to the bottom and find phpMyAdmin and click.
now on the page that opens on the left you will see a dropdown box click the arrow and select your forum and page will open. along the top you will see a tool bar
Structure SQL Search Query Export Import Operations CLCIK THE Import link, on the page that opens you will see a text box with Location of the text file on one side and a Browse button on the other, click the browse button. find the sql file you saved earlier press ok, that file location is now in the text box. Now in the bottom right corner there is a GO button click it, wants it has finished close down and logo out of cpanel and check the admin of your forum you will see the date has changed.
Do this first then come back and i will post how to change the user start date. have to go shop now so will be about a hour.