Thanks to Michael Hofmann there is now an updated version of the deskbar calculator suitable for the 2.20 release of the deskbar applet. You can find it as a tarball here, or if you just want to drag it directly into the deskbar preferences pane, try this link. If you have an older version of the deskbar applet, you should still be using version 1.6, which is still available, just a little further down the page.
I must apologise to Michael for taking so long to release this update. He did all the hard work for me two weeks ago, but I was slow about updating my test scripts (which the API changes also broke).