Example of how to parse a custom shortcode ([DATETIME_ID]) added to the EE_Datetime_Shortcodes library.

ee_register_new_custom_messages_shortcodes_parser.php

<?php
/*  
 * This filter allows you to hook in to the shortcode parser, check for a shortcode added above and return a value for it using the data passed to the parser.
 * $parsed are the current values being parsed.
 * $shortcode is the current shortcode passed to the parser.
 * $data is the current data available, this can be different types of objects depending on the parser.
 * $extra_data is a collaction of various data available within the messages system.
 */
function ee_register_new_custom_messages_shortcodes_parser( $parsed, $shortcode, $data, $extra_data, EE_Shortcodes $lib ) {
	
	//Check for the datetime shortcodes library as that's one of the libraries we added a custom shortcode to above.
	//also check that $data is the expected object (in this case an EE_Datetime as its for the EE_Datetime_Shortcodes library)
	if ( $lib instanceof EE_Datetime_Shortcodes && $data instanceof EE_Datetime ) {
		//Then check if we are parsing one of our custom shortcodes above.
		if ( $shortcode === '[DATETIME_ID]' ) {
			
			//Do whatever you need to do here and return the value for that specific datetime.
			//In this example it simply returns the ID of the related EE_Datetime.
			return $data->ID();
		}
	}

	//If not within the correct section, or parsing the correct shortcode,
	//Return the currently parsed content.
	return $parsed;
}
add_filter( 'FHEE__EE_Shortcodes__parser_after', 'ee_register_new_custom_messages_shortcodes_parser', 10, 5 );