Currently, we don't have the PHP:Calendar module installed on the Sonic.net web cluster.
I did find something that may work for you - there's an alternate set of functions available in the PEAR Date class. I found
this forum post to be a good resource.
I encourage you to try playing around a bit on shell.sonic.net to see if it's possible to install the PEAR module for your site. You can check out the installation instructions
here.