A trivial WordPress plugin to add a meta box with the number of days since a certain date to the Post editing page. This was a quick knock-up job and has by no means been thoroughly tested. However, it might give someone some clues if they're looking to add a Meta box on a plugin settings page, for example, and I'm personally using it on a site and it doesn't seem to have caused any problems.
If you install this plugin (grab the repository and throw it under the directory name "days-since-counter" in wp-content/plugins) you should find a settings page under Settings->Days Since Counter in the WordPress admin interface. Set a date there, and then on your New Post or Edit Post pages, you'll find a "meta box" appears with the number of days between that date and the post's date.
I found this useful when doing a photography "365" project where I wanted to know how many days I'd been doing the project whenever I posted my new daily image.