No, at this time offline mode is not supported. If you don't have an internet connection you won't be able to log into your TrackingTime account or track time while offline. We're currently evaluating this feature and have no specific release plans yet.
That being said, adding your time entries manually for you or your employees, in case you have been without connection for some time, is easy. Please refer to this article to learn how to manually add and edit your time entries.