Chrono is a simple Gaia OpenWebApp coded as example in "Hack On FirefoxOS" talk during local LinuxDay2012 event.
You can use it as a normal web app on Firefox (optionally pinned as App Tab), install it as Desktop Web App from your Firefox Nightly or as Mobile Web App on Firefox for Android Nightly and FirefoxOS. It will work even offline thanks to the manifest.appcache generated by volo-appcache.
Chrono uses volo as devtool to speed up development and to generate appcache manifest, Gaia UI Building Blocks to style the app as a FirefoxOS native application and RequireJS to split javascript code into modules and build a single production file.