The "Countdown" application uses some older infrastructure and conventions which should be relatively straightforward to upgrade:

• The Countdown controllers do a bit of "C{$id}" / 'C'.$thing->getID() to build C123 monograms for countdowns. Replace these with $countdown->getMonogram().
• PhabricatorCountdownView (and maybe elsewhere) manually builds the URI for a countdown, but should call $countdown->getURI().
• Find anything else using the old /countdown/123/ URIs and replace them with $countdown->getURI(), then remove the route for /countdown/123/ from CountdownApplication.
• Convert Countdown to ModularTransactions. Examples: D17610, D17402
• Add countdown.search and countdown.edit API methods. T12006 (from the "Locations" scenario) is fictional, but a good template for this.