I need help handling form posting

Hi all: In my quest to make things seem easier for the user to understand, I'm trying to retrieve the time that I start posting a very large file via a form post. The form action sends it back to itself, which is fine. It took me some time to figure out that NOTHING happens on the client until the form has been completely transmitted to the server. In my particular case, this can be a long time, anywhere from 15 to 30 minutes. I'd like to be able to notify the user of when the file upload actually began. Is there a way that I can intercept the click of the "Upload" button, have it update a field (probably a hidden one) with a date/time stamp, and then have that value included in the $_POSTed values? I'm thinking (fearing, rather) that this is probably all off-topic and probably is best addressed with some type of JavaScript solution, but it's late, I'm not thinking clearly, and I've been doing data conversions all day, which is a sure-fire way to get me emulating some Romero zombies. Any help you can spare will be greatly appreciated. Jon