Create monthly archive for parent category

Alright, I solved it:

I queried the year and month of the page we’re on with:

$year     = get_query_var('year');
$monthnum = get_query_var('monthnum');

And then I added date parameters to each WP_Query:

        $col1 = new WP_Query(
        array(
            'year'                      => $year,
            'monthnum'                  => $monthnum,
            'posts_per_page'            => 1,
            'cat'                       => 31
            )
        );

This allows me to split the departments of the ‘Magazine’ archive page into different areas, rather than have them mixed in one loop.