We are starting planning activities for next year and we built a “perpetual” planner, which we instantiated for the following 15 months (Sep 2014 — Dec 2015).
The “compulsory” screenshot:
It is a LibreOffice spreadsheet, which can be downloaded here: PerpetualPlanner.ods. Notice that there are no formulas to compute the days: this could be a nice extension, if you feel to do so!
Distributed under the conditions of the MIT license.