I liked the idea of Sotak's calendar icons, but the colours didn't fit in with my theme, and I wanted the text to be a little easier to read, and to include the year.
So I made my own:
Rather than an icon for each month, this technique uses just 1 background image (.png and psd attached) and a slightly modified template.php. Everything else can be styled with CSS.
With a bit of imagination, and a couple of tweaks to the function, and a few lines of CSS these icons can be very flexible. Different colour for articles over a year old? No problem. Colour coded by taxonomy? Sorted.