double_meta_query.php

<?php
$query_args = array(

	'meta_query' => array(
		// relation between time query and online query
		'relation' => 'OR',

		
		// time query
		array(
			// relation for time query
			'relation' => 'AND',
			array(
				'key'     => 'start_time',
				'value'   => '20191018100000',
				'compare' => '>=',
			),
			array(
				'key'     => 'end_time',
				'value'   => '20191018120000',
				'compare' => '<=',
			),
		),

		
		// online query
		array(
			'key'     => 'online',
			'value'   => 'true',
			'compare' => '=',
		),
	), // end meta_query
);

Almost; except that start_time and end_time are time values, not full datetime values, so, the value would look more like 12:00:00 or 13:59:48, and the type parameter for those 2 queries is set to TIME.

So:

array(
	'key'     => 'start_time',
	'value'   => '20191018100000',
	'compare' => '>=',
),

becomes:

array(
	'key'     => 'start_time',
	'value'   => '10:00:00',
	'compare' => '>=',
	'type' => 'TIME', 
),

This seems to work:

$meta_query['time-range'] = $meta_query['time-range'] = array(
	'relation' => 'AND',
	array(
		'key'     => 'hsucs_start_time',
		'value'   => array( '00:00:01', $t['start'] . ':00' ),
		'compare' => 'NOT BETWEEN',
	),
	array(
		'key'     => 'hsucs_end_time',
		'value'   => array( $t['end'] . ':00', '23:59:59' ),
		'compare' => 'NOT BETWEEN',
	)
);