Default date doesn't work

Problem/Motivation

According to the UI in month display, for the default date:

Default starting date of this calendar, in any machine readable format.
Leave empty to use the date of the first result out of the first selected Date filter above.
NB: The first result is controlled by the Sort order on this View.

It seems like this is not working - the view is always showing the current date.

Steps to reproduce

1. Add a date field to article - "Posted date"
2. Generate articles using devel generate
3. Create a calendar view with month display - no default date
4. Sort by Posted date, ascending

If you select "Display default View results" you can see generated items with Posted dates from the prior year, sorted in chronological order.

With this configuration, the calendar display should render with the first item's month.

Proposed resolution

Calendar View uses query parameters for the current date and pagers.

Does it make sense to add a contextual filter to set the default date instead?
This would make it easier to link to the displays.
It may make sense to use the contextual filter values as pager parameters too.

I think this is how the Calendar module works.

Remaining tasks

User interface changes

API changes

Data model changes