Complex WordPress meta query by start and end date (custom meta fields)

event_query.php

<?php

/**
 * Complex WordPress meta query by start and end date (custom meta fields)
 * Intended for use on the `pre_get_posts` hook.
 * Caution; this makes the query very slow - several seconds - so should be 
 * implemented with some form of caching.
 *
 * mark@sayhello.ch 22.10.2019, based on code from 201 onwards
 */

add_action('pre_get_posts', [$this, 'onlyFutureEntries']);

…

public function onlyFutureEntries($query)
	{
		if (!is_admin() && isset($query->query['post_type']) && $query->query['post_type'] === 'sht_event') {
			$query->set('orderby', 'meta_value');
			$query->set('order', 'ASC');
			$query->set('meta_key', 'start_date');

			$meta_query = (array) $query->get('meta_query');

			$meta_query[] = [
				'relation' => 'OR',
				[
					// Start date and end date are empty
					'relation' => 'AND',
					[
						'relation' => 'OR',
						[
							'key'     => 'start_date',
							'value' => '',
							'compare' => '='
						],
						[
							'key'     => 'start_date',
							'compare' => 'NOT EXISTS'
						]
					],
					[
						'relation' => 'OR',
						[
							'key'     => 'end_date',
							'value' => '',
							'compare' => '='
						],
						[
							'key'     => 'end_date',
							'compare' => 'NOT EXISTS',
						]
					]
				],
				[
					// Start date >= today and end date empty
					'relation' => 'AND',
					[
						'key'     => 'start_date',
						'value' => date('Y-m-d'),
						'compare' => '>=',
						'type' => 'DATE'
					],
					[
						'key'     => 'end_date',
						'value' => '',
						'compare' => '='
					]
				],
				[
					// Start date <= today and end date >= today
					'relation' => 'AND',
					[
						'key'     => 'start_date',
						'value' => date('Y-m-d'),
						'compare' => '<=',
						'type' => 'DATE'
					],
					[
						'key'     => 'end_date',
						'value' => date('Y-m-d'),
						'compare' => '>=',
						'type' => 'DATE'
					]
				]
			];

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

Hello Mark, would you have a minute to spare to help me with implementing your snippet to my case, I got myself in a real struggle. Thanks.

What's the problem?