Here is a first commited patch relevant to issue #88217. Here are its main features:

* Replace the "duration" select box by a "Closing date" widget.
* Add a "Start date" widget.
* Does not change the current fields in the DB whatsoever.
The plan is to reintegrate the duration as an option later on (like, you'd be able to choose between a closing date or a duration). This patch merely handles the saving/loading of those values: it still does not affects the behavior of the decision.