This idea has been proposed before. The previous thread from 2013 is now closed to new comments, so I'm opening up a new thread to track interest in the idea.
The lack of this feature is one of the main shortcomings of Hover. The more domains you have, the more tedious keeping track of the renewals becomes. Consolidating them all to the same date each year would be tremendously helpful. Other registrars offer this feature.
The proper way to do it is to permit users to renew ahead of the actual expiration date, and to provide a pro-rating of the cost of the next registration based on remaining days in the current registration. That way people can get all of their renewal dates combined and only have to deal with expiring domains once per year.
Please comment if you also would like this feature.