I have created field type "Timestamp" in my Drupal 8 node. Field is not required. In field settings default value, there is HTML 5 datetime picker, in which you can select default date/time or leave values as on picture:
But the Drupal behavior seem to be little bit strange - when you select default date, this value is set on node save (this is correct). When you leave field blank (values as on picture) it saves field with time of form submission (it is stated in field comment, so it is not a bug)... But how to disable default value, when you just want to leave field blank as it is possible with all other fields (e.g. text, number, ...)? I do not see any settings for this in this field configuration. Also this is not logical, beucase when you leave other fields blank, they are really blank. When you leave this field type blank, it still saves value (some additional checkbox will be more logical)?