Archive by custom post type and custom date field

Question

I have created a custom post type called 'events' and custom date field called 'event_date' .

I was able to create archive list for custom post type using this awesome plugin

This plugin allowed me create archives list url in this format : www.example.com/2015/11/events/

Till this point every thing worked perfectly.

But I wanted to created archives by 'event_date' unlike usual by published date, so made some changes in the above mentioned plugin and was able to create an archive list by 'event_date'.

I am unable to view posts when I click on the link : www.example.com/2016/09/events/

Now when I am writing this question the month is november so for events in november it returns correct posts.

I have used pre_get_post to get event by 'event_date' but it still returns posts by published date. Here is my code to get the posts:

function custom_events_query( $query ) {

    if (  $query->is_main_query() && $query->is_archive && !is_admin()&& is_post_type_archive('events') && $query->query_vars['m']!='') {

        $query->set( 'post_type', 'events' );
        $query->set( 'meta_key', 'event_date' );
        $query->set( 'orderby', 'meta_value' );
        $query->set( 'order', 'ASC' );
        $query->set( 'post_per_page', 10 );
        unset($query->query_vars['year']);
        unset($query->query_vars['monthnum']);
        unset($query->query_vars['day']);

        $meta_query = array(
            array(
                'key' => 'event_date',
                'value' =>  array(date('Y-m-d',strtotime($query->query_vars['m'].'01')),date('Y-m-d',strtotime($query->query_vars['m'].'31'))) ,
                'compare' => '>=',
            )
        );

        $query->set( 'meta_query', $meta_query );

    }

    return $query;
}
add_filter( 'pre_get_posts', 'custom_events_query' );

Why does it still return posts by published date when I have

unset($query->query_vars['year']); 
unset($query->query_vars['monthnum']);

Here is a link to the output which i get when trying to open: http://www.example.com/2016/08/events/

I have started using wordpress very recently and I am not sure about using pre_get_post correctly. It would be great if you could help me with this final step. Thanks in advance. Forgive me for trying to explain my entire journey through events, I just wanted to make sure I was doing things correctly.

Answer 1

Thanks to Milo for helping me through this. Here is the code that worked for me.

function custom_events_query( $query ) {

    if (  $query->is_main_query() && $query->is_archive && !is_admin()&& is_post_type_archive('events') && $query->query_vars['m']!='') {

        $query->set( 'post_type', 'events' );
        $query->set( 'meta_key', 'event_date' );
        $query->set( 'orderby', 'meta_value' );
        $query->set( 'order', 'DESC' );
        $query->set( 'post_per_page', 10 );
        unset($query->query_vars['year']);
        unset($query->query_vars['monthnum']);
        unset($query->query_vars['day']);

        $meta_query = array(
            array(
                'key' => 'event_date',
                'value' =>  array(date('Y-m-d',strtotime($query->query_vars['m'].'01')),date('Y-m-d',strtotime($query->query_vars['m'].'31'))) ,
                'compare' => 'BETWEEN'
            )
        );
        unset($query->query_vars['m']);
        $query->set( 'meta_query', $meta_query );
    }

    return $query;
}
add_filter( 'pre_get_posts', 'custom_events_query' );

All I had to do is unset the query_vars['m']. Since I needed it to construct my meta_query I unset it only after I was done using it i.e just before $query->set( 'meta_query', $meta_query );

I hope this helps someone.