Yes! Marvin works offline on every supported device (desktop apps, web app, mobile).

If you don't have Internet connection you can create tasks, check off tasks and do almost everything else in Marvin (except time tracking, see below).

You can load the Marvin app in your browser (app.amazingmarvin.com) and then go offline and keep using Marvin. But you need to avoid reloading the page or Marvin can't be loaded again since you don't have a connection to our server that sends out the app.
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If you work offline a lot it is safer to just download the entire app to your computer (via our desktop apps) and use those.

Time tracking is currently the only feature that requires you to be connected to the Internet in order to work. 

Time tracking runs over our server to reduce conflicts.