I have an old drupal6 site and I need to upgrade it to drupal8. I decided to do it in 2 steps (d6 -> d7 -> d8), because I thought this way I can manage the things better.
I removed those fields that I don't need any more or not supported by drupal7 or drupal8 to make the whole process easier.
I changed the core of my old drupal6 to drupal7 and installed the necessary modules together with the migration modules:
date migration example,
I followed this guide: Migrating D6 Content Construction Kit (CCK) to D7 Fields
I have a list of the invalid fields that have to be converted in the
content migration UI. I have 2 content types where I use a
date field with date repeat options. The widget name is
select list with repeat options and I see this error message next to them:
Missing widget: The 'date_select_repeat' widget is not available for the field_date field, it will be set to the default widget.
I accepted it and migrated them. Then I upgraded my site to drupal8. I see a complete mess in drupal8. It added hundreds random dates to every content and there is no option to activate the date repeat option on this date field, because it contains content. So I have to delete and recreate this field again and setup every content except you have an idea. I can't tell you whether the date was right in the drupal7 site (strongly believe not) after the migration, because I reverted this field and so far it disappeared from the affected content types.