Custom Wordpress Script For Modifying Post Data Needed, Please.

Hello, Basically I need a simple bit of customized code for a worpress site. I need a script that will make a simple captcha appear at the bottom of my site, at the footer. The captcha should appear only for logged-in users. If a user fills the captcha, it should re-appear after 24 hours - every user account gets a unique captcha - I mean that if one user fills it up and clicks ok, it doesn't disappear altogether for all users, but only for that user and only for 24 hours - regardless of him/her logging in and out druing this time. Maybe use a cron job... If the user doesn't fill-up the captcha but just surfs the site - the captcha simply stays there at the footer. Non-registered users don't see any captchas and have nothing to do with that. What the captcha is for: Let's say a user that is registered on my wordress site has posted three posts since he became a member. Whenever he fills and sends the captcha, I need the script to make a modification his most recent post of the three. The modification is that the script will get that post's "date modified" field (not "date created", and if there is no "date modified, just create an entry there) and insert that moment's (when the user clicks ok after filling the captcha) server time (in the wordpress time format) into that field. I'll gladly answer any questions... Thank you very much!