Date-only range expands on save

The problem is in smart_date/src/Plugin/Field/FieldWidget/SmartDateOnlyWidget.php, lines 53 to 59:

// Force to all day.
      if (!empty($item['value']) && $item['value'] instanceof DrupalDateTime) {
        $item['value']->setTime(0, 0, 0);
      }
      if (!empty($item['end_value']) && $item['value'] instanceof DrupalDateTime) {
        $item['end_value']->setTime(23, 59, 0);
      }

If I comment out that section the dates are not changed.

However, I don't know why this causes the problem. It appears that before being reset, the time part of the field is set to the current time, so the duration is an exact number of days. "Force to all day" increases that duration. Maybe the duration is being remembered somewhere and the parent::massageFormValues() that follows adjusts the beginning date to be (original end date) minus duration, the adjusts the end date to (original beginning date) plus duration? The interval always increases by two days.

Yes, I think you're right. Setting the tine zone to the system default (as is done in SmartDateWidgetBase.php) seems to solve the problem. It might be simpler to eliminate the "force to all day" code, but that might have other consequences. I found that the time was initially set to the current time; that seems to happen both with field setting default as "current date" and no default value.

Sorry, careless copy/past in the previous patch - the second test should be on 'end_value', not 'value'. (I noticed that when I entered an invalid date and it threw an error.) See the attached revision (3303444-5).

Just as a note related to this issue, the code makes an interpretation that might not be expected. If the start and end dates are the same, the start value and end value in the date field will NOT be equal, but will rather be just under 24 hours apart. (And stripping the time might not result in equal dates, depending on time zone considerations.) Making the interval between start and end be a full day often makes sense, but it could be confusing programmatically that the values are not the same, when the widget displays the same dates.