How to set end date relative to start date in date range field Is it possible to set the end date relative to the selected start date in a D8 date range field?
I have a date range field that I use to specify an accounting period. The end date should now be put to one year minus one day from the selected start date. For example, the start date 2016-07-01 will give the end date 2017-06-30.
How is this possible?
The current widgets in core for the Datetime Range module do not allow this.

You would need to create a custom widget for this.  Off the top of my head, I would create one that extends DateRangeWidgetBase() and mostly mirrors what DateRangeDefaultWidget() does, except to collect a relative period in a plain text field for the end date.  Then, add a massageFormValues() function to turn the relative date into an absolute one, and then call parent::massageFormValues().  You may also be able to do this with a #process callback on the end date element, but I forget which of the two runs first during form processing.