allow Tribe Events feeds to be fetched by post_date, and include start date

the-events-calendar.php

<?php

/**
 * Custom functionality for the-events-calendar.
 */

function cactec_is_tec_feed() {
	if ( ! class_exists( 'Tribe__Events__Main' ) ) {
		return false ;
	}

	if ( ! is_feed() || Tribe__Events__Main::POSTTYPE !== get_query_var( 'post_type' ) ) {
		return false;
	}

	return true;
}

function cactec_init_feed_params( $query ) {
	if ( ! cactec_is_tec_feed() ) {
		return;
	}

	if ( ! isset( $_GET['orderby'] ) || 'post_date' !== $_GET['orderby'] ) {
		return;
	}

	remove_filter( 'get_post_time', array( 'Tribe__Events__Templates', 'event_date_to_pubDate' ), 10, 3 );
	remove_filter( 'posts_orderby', array( 'Tribe__Events__Query', 'posts_orderby' ), 10, 2 );
}
add_action( 'pre_get_posts', 'cactec_init_feed_params', 999 );

/**
 * Include the event start date in feeds.
 *
 * Since we're replacing the pubDate with the actual publication date in some
 * cases, we must make the start date available via a different element.
 */
function cactec_include_start_date() {
	if ( ! cactec_is_tec_feed() ) {
		return;
	}

	?>
	<cunyAcademicCommons:eventStart><?php echo tribe_get_start_date( get_the_ID(), true, Tribe__Date_Utils::DBDATETIMEFORMAT ); ?></cunyAcademicCommons:eventStart>
<?php
}
add_action( 'rss2_item', 'cactec_include_start_date' );

function cactec_include_cac_namespace() {
	if ( ! cactec_is_tec_feed() ) {
		return;
	}

	?>
xmlns:cunyAcademicCommons="https://commons.gc.cuny.edu/rss/modules/cunyAcademicCommons/"
<?php
}
add_action( 'rss2_ns', 'cactec_include_cac_namespace' );