Activity import ignores time component of activity_date_time if the import file includes seconds and always ignores time for custom date fields

e.g. for activity_date_tinme 2021-01-02 10:11 imports correctly, but 2021-01-02 10:11:12 imports as 2021-01-02 00:00.

In master but also goes back to at least 5.24.

For custom fields of type date it seems to always ignore time.