Change date format on editing I'm using a date field on my custom type, if I use "select" widget for node editing, I can choose only between "YMD","MDY" and "DMY" formats.

Is there a way to add custom formats? I need an "only year" format.
By default, no.  The relevant code is in \Drupal\datetime\Plugin\Field\FieldWidget\DateTimeDatelistWidget::formElement().

What you want is usually called a "partial date", which is a bit of an oxymoron.  In Drupal, datetime fields are stored as DrualDateTime objects, which derive from \DateTime so have both a full date and a proper time and timezone associated with them.  

A partial date (year, year+month), has to make assumptions about the rest of the parts.  For example, the date-only field currently sets the time to noon UTC.  A year only field would have to do something similar.

You may be able to create your own widget for the datetime field to do this.  You could take DateTimeDatelistWidget as a starting point, and trim it down to be just the year.  Then, you would need a masssageFormValues() to set the rest of the parts and then call the parent method (because DateTimeWidgetBase::masssageFormValues() does stuff, too).  This may or may not work, it is just the idea that popped into my head.

There is an open issue about this, https://www.drupal.org/node/2703037, that may or may not be implemeneted.