This plugin can be used to add custom shortcodes to the EE3 confirmation emails. Currently it adds the [reg_date] shortcode which parses to the date and time that the user laced the registration.

ee-site-specific-plugin.php

<?php
/*
Plugin Name: Event Espresso site specific functions
Description: Add custom functions for Event Espresso to this plugin.
/* Begin Adding Functions Below This Line; Do not include an opening PHP tag as this sample code already includes one! */

//Add a custom shortcode for use within the emails.
function espresso_custom_email_shortcode( $shortcodes ) {
	$shortcodes[] = "[reg_date]";

	return $shortcodes;
}
add_filter('filter_hook_espresso_post_replace_shortcode_search_values', 'espresso_custom_email_shortcode');

//Add the value for the custom shortcode we just added above.
function espresso_custom_email_shortcode_value( $shortcode_values, $data ) {
	$shortcode_values[] = $data->attendee->date;

	return $shortcode_values;
}
add_filter('filter_hook_espresso_post_replace_shortcode_replace_values', 'espresso_custom_email_shortcode_value', 10, 2);