I'm just starting out with D8 and I'm trying to migrate a D7 datetime with an end date to a D8 daterange field, but the field migration `migrate_plus.migration.upgrade_d7_field.yml` always creates a datetime field rather than a daterange:
DESCRIBE node__field_display_dates;
+---------------------------+------------------+------+-----+---------+-------+
| Field                     | Type             | Null | Key | Default | Extra |
+---------------------------+------------------+------+-----+---------+-------+
| bundle                    | varchar(128)     | NO   | MUL |         |       |
| deleted                   | tinyint(4)       | NO   | PRI | 0       |       |
| entity_id                 | int(10) unsigned | NO   | PRI | NULL    |       |
| revision_id               | int(10) unsigned | NO   | MUL | NULL    |       |
| langcode                  | varchar(32)      | NO   | PRI |         |       |
| delta                     | int(10) unsigned | NO   | PRI | NULL    |       |
| field_display_dates_value | varchar(20)      | NO   | MUL | NULL    |       |
+---------------------------+------------------+------+-----+---------+-------+
I've been banging my head over this for days and can't seem to make any progress. The secret sauce required to migrate to create and populate a daterange field has eluded me.
I have mangaged to get the migration to work once with some judicious hacking, by editing the content type in the UI, and manually editing the `config` table to implemement the code in 
            
              this comment
              
              📌
              FormatDate process plugin
                Fixed
              
             (Using the config import/export failed oh so spectacularly).
Any guidance would be much appreciated.
Installed core and contrib are latest release at time of writing.
Destination
Drupal 8.6.3
drush pml | grep -E "(date|migrat)" | grep Enabled
  Field types   Datetime (datetime)                   Enabled    8.6.3         
  Field types   Datetime Range (datetime_range)       Enabled    8.6.3         
  Migration     Migrate (migrate)                     Enabled    8.6.3         
  Migration     Migrate Drupal (migrate_drupal)       Enabled    8.6.3         
  Migration     Migrate Drupal UI (migrate_drupal_ui) Enabled    8.6.3         
  Migration     Migrate Plus (migrate_plus)           Enabled    8.x-4.0       
  Migration     Migrate Tools (migrate_tools)         Enabled    8.x-4.0       
  Migration     Drupal Upgrade (migrate_upgrade)      Enabled    8.x-3.0-rc5   
Source
Drupal 7.61
drush pml | grep -i date | grep Enabled
 Date/Time      Date (date)                   Module  Enabled   7.x-2.10
Source Table
DESCRIBE field_data_field_display_dates;
+----------------------------+------------------+------+-----+---------+-------+
| Field                      | Type             | Null | Key | Default | Extra |
+----------------------------+------------------+------+-----+---------+-------+
| entity_type                | varchar(128)     | NO   | PRI |         |       |
| bundle                     | varchar(128)     | NO   | MUL |         |       |
| deleted                    | tinyint(4)       | NO   | PRI | 0       |       |
| entity_id                  | int(10) unsigned | NO   | PRI | NULL    |       |
| revision_id                | int(10) unsigned | YES  | MUL | NULL    |       |
| language                   | varchar(32)      | NO   | PRI |         |       |
| delta                      | int(10) unsigned | NO   | PRI | NULL    |       |
| field_display_dates_value  | datetime         | YES  |     | NULL    |       |
| field_display_dates_value2 | datetime         | YES  |     | NULL    |       |
+----------------------------+------------------+------+-----+---------+-------+