Change an hidden value using greasemonkey, on submit form

Hello. It's a content form page, that include a time checker added into a hidden value. Here's the hidden value : <input type="hidden" name="time" value="1306410024"> The VALUE is the full unix time, created when i open the content form. When i submit this content form, it check if XX seconds are passed (compared to the VALUE on the HIDDEN INPUT). If XX seconds are passed, the content form is blocked and refuse to add what i want to submit. I need a javascript (wich work with Greasemonkey firefox plugin) that can, on submit the content form, automatically change the unix time VALUE to the present one (to "block" the XX time limitation) (simulate the thing i have just opened/submit the content form). I assume it's my first project here, so Pre-pay is 50% is available. Thanks in advance.