I did them. I wanted to keep it quiet so I wouldn't get inundated with requests as I don't have time to do loads, but I'm happy to share the principal of them so you can get others to do them.
Each one is custom programmed to get the text in the right place, although the years, months, weeks and days is done from a central function. They're done in PHP, but the file needs a .png (or other image file) extension to work as a sig (png means they don't mess up other images on any websites on the server) (htaccess does it).
You need to write script to calculate the years, months, weeks and days. I've spent 3 years tweaking mine so it's not something I can give away unfortunately. I use timestamps. I store each of the years, months weeks and days in an array. You also need to beware of the GMT / BST time changes
You then use GD image library (another PHP thing) to get the background image and display the text - that's the easy bit
You can't display flash / html in sigs - PHPBB strips out anything but basic HTML. You can't even display an image with a php extension hence the need for htaccess to change the extension.
I'm sorry I can't do them for everyone
I feel really guilty now