Some client need to get all encounter fisnishing before and exact date. Searching by date looks quite unprecise.
Also there is no way to _sort by end/start.

I have two use cases:
- search for all encounter who have missing end-date, OR end-date between x and y
- sort them by start date.

Encounter?date=2019 // no broken change

Encounter?start-date=2019

Encounter?start-date:lt=2020-02&start-date:gt=2017

Encounter?end-date:missing=false

Encounter?_sort=-start-date

the proposal adds start-date and end-date as valid search datetime parameters for resource Encounter (and any in the same case with period type )