Does Chandler need Cosmo to run?
Chandler is an independent desktop client. You can keep all your data locally, or synchronize with a variety of WebDAV and CalDAV servers if you prefer. So, no, you don’t need to use Cosmo to use Chandler. If you do choose to synchronize with a Cosmo server, you’ll have access to a few nifty features like a task dashboard that mirrors that found in Chandler.