Occasionally you may need to change up the data as it is migrating from your D6 / D7 site to your D8 site.  This is common especially in the case of migrating a Date field.

You can alter a data by extending the ProcessPluginBase and using the transform method.

1. You should of Run the a custom migration and created a module.  We will need to edit one of the config YML files.
2. In this case we are changing the node import which is usual identified like: migrate_plus.migration.upgrade_d7_node_YOURTYPE.yml

3. We are going to change our date field in the yml from:

   field_CUSTOM_DATE: field_CUSTOM_DATE

   1. We are changing the process handler to:

      field_CUSTOM_DATE: plugin: fix_date source: field_CUSTOM_DATE

4. d