app-exclude-specific-hours.php

<?php
/**
* Plugin Name: [Appointments +] - Exclude specific hours
* Plugin URI: https://premium.wpmudev.org/
* Description: A helper snippet to exclude specific times from specific days
* Author: Panos Lyrakis @ WPMUDEV
* Author URI: https://premium.wpmudev.org/
* License: GPLv2 or later
*/

if ( ! defined( 'ABSPATH' ) ) {
    exit;
}


add_filter( 'app-is_busy', function( $is_busy, $period ){

	/**
	* ISO-8601 numeric representation of the day of the week (added in PHP 5.1.0)
	* Days format : 
	* 	1 = Monday
	* 	2 = Tuesday
	* 	3 = Wednesday
	* 	4 = Thursday
	* 	5 = Friday
	* 	6 = Saturday
	* 	7 = Sunday
	*/

	$exclusions = array();

	// Thurday 11:00 to 11:30 is unavailable
	$exclusions[] = array( 
		'day' 		=> 4,
		'start'		=> "11:00",
		'end' 		=> '11:30',
	);

	// Thurday 13:30 to 15:00 is unavailable
	$exclusions[] = array( 
		'day' 		=> 4,
		'start'		=> "13:30",
		'end' 		=> '15:00',
	);

	// Friday 12:00 to 14:40 is unavailable
	$exclusions[] = array( 
		'day' 		=> 5,
		'start'		=> "12:00",
		'end' 		=> '14:40',
	);

	$period_start 			= $period->get_start();
	$period_end 			= $period->get_end();
	$period_start_time 		= strtotime( date( "H:i", $period_start ) );
	$period_end_time 		= strtotime( date( "H:i", $period_end ) );
	$current_day_num  		= date( 'N',  $period_start );

	// Quicly skip check for irrelevant days
	$excluded_day_nums = array_unique( array_column( $exclusions, 'day') );

	if ( ! in_array( $current_day_num, $excluded_day_nums ) ) {
		return $is_busy;
	}

	// Check days and times
	
	$current_day_exclusions = array();
	foreach( $exclusions as $key => $exclusion ) {

		if ( $current_day_num != $exclusion[ 'day' ] ) {
			continue;
		}

		$excluded_start_time = strtotime( $exclusion[ 'start' ] );
		$excluded_end_time = strtotime( $exclusion[ 'end' ] );

		if ( 
			( $period_start_time >= $excluded_start_time && $period_start_time  <= $excluded_end_time )
			// ||
			//( $period_end_time >= $excluded_start_time && $period_end_time  <= $excluded_end_time )
		){
			$is_busy = true;
			break;
		}

	}

	return $is_busy;
}, 20, 2 );