Your first plugin: a shortcode

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/**
 * Plugin Name:       Sunrise Opening Hours
 * Description:       Shows the bakery's opening hours and whether it's open now, with the [sunrise_hours] shortcode.
 * Version:           1.0.0
 * Requires at least: 6.7
 * Requires PHP:      7.4
 * Author:            Sunrise Bakery
 * License:           GPL-2.0-or-later
 * Text Domain:       sunrise-hours
 *
 * @package SunriseHours
 */

// Stop the file being run directly from a browser.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Opening hours: day number (0 = Sunday) => array( opens, closes ) in 24-hour time.
 * Days not listed are closed.
 */
function sunrise_hours_schedule() {
	$schedule = array(
		1 => array( '07:00', '20:00' ),
		2 => array( '07:00', '20:00' ),
		3 => array( '07:00', '20:00' ),
		4 => array( '07:00', '20:00' ),
		5 => array( '07:00', '20:00' ),
		6 => array( '07:00', '20:00' ),
	);
	// Let other code (or a child theme) change the hours without editing this plugin.
	return apply_filters( 'sunrise_hours_schedule', $schedule );
}

/**
 * Is the bakery open at this moment? Uses the site's time zone (Settings > General).
 *
 * @param DateTimeImmutable|null $now Time to check; defaults to now.
 * @return bool
 */
function sunrise_hours_is_open( $now = null ) {
	$now      = $now ? $now : new DateTimeImmutable( 'now', wp_timezone() );
	$schedule = sunrise_hours_schedule();
	$day      = (int) $now->format( 'w' );
	if ( empty( $schedule[ $day ] ) ) {
		return false;
	}
	$time = $now->format( 'H:i' );
	return $time >= $schedule[ $day ][0] && $time < $schedule[ $day ][1];
}

/**
 * [sunrise_hours] shortcode handler. Shortcodes must return their HTML, not echo it.
 *
 * @param array $atts Shortcode attributes, e.g. [sunrise_hours title="When to visit"].
 * @return string
 */
function sunrise_hours_shortcode( $atts ) {
	$atts = shortcode_atts(
		array( 'title' => __( 'Opening hours', 'sunrise-hours' ) ),
		$atts,
		'sunrise_hours'
	);

	$days     = array( 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday' );
	$schedule = sunrise_hours_schedule();
	$open     = sunrise_hours_is_open();

	$html  = '<div class="sunrise-hours">';
	$html .= '<h3>' . esc_html( $atts['title'] ) . '</h3>';
	$html .= '<p class="sunrise-hours__status">' . esc_html( $open ? __( 'Open now', 'sunrise-hours' ) : __( 'Closed right now', 'sunrise-hours' ) ) . '</p>';
	$html .= '<ul>';
	foreach ( array( 1, 2, 3, 4, 5, 6, 0 ) as $d ) {
		$hours = empty( $schedule[ $d ] ) ? __( 'Closed', 'sunrise-hours' ) : $schedule[ $d ][0] . '–' . $schedule[ $d ][1];
		$html .= '<li>' . esc_html( $days[ $d ] . ': ' . $hours ) . '</li>';
	}
	$html .= '</ul></div>';

	return $html;
}

add_action(
	'init',
	function () {
		add_shortcode( 'sunrise_hours', 'sunrise_hours_shortcode' );
	}
);