How do multiple date queries differ between Craft 2 and 3

Question

After spending a lot of time solving this for myself I thought I'd share it. In my Craft 2 template I had this query:

{% set result = craft.entries({
    section: 'events',
    eventDate: ['and', '<' ~ now.w3c(), '> ' ~ now | date_modify('-6 days')],
    results: ':notempty:',
    order: 'eventDate desc',
    with: ['imageGallery'],
    limit: 3
}) %}

Answer 1

My Craft 3 version is:

{% set result = craft.entries()
    .section('events')
    .eventDate( ['and', '<' ~ now|atom, '>' ~ now|date_modify('-6 days')|atom ] )
    .results(':notempty:')
    .orderBy('eventDate desc')
    .limit(3)
    .with(['imageGallery'])
    .all()
%}

Note the extra |atom after the date_modify('-6 days')