astra-post-date-updated.php

<?php // don't copy this line in your code

/**
 * Display only last modified date in the post metadata.
 *
 * @param String $output Markup for the last modified date.
 * @return void
 */
function your_prefix_post_date( $output ) {
	$output        = '';
	$format        = apply_filters( 'astra_post_date_format', '' );
	$modified_date = esc_html( get_the_modified_date( $format ) );
	$modified_on   = sprintf(
		esc_html( '%s' ),
		$modified_date
	);
	$output       .= '<span class="posted-on">';
	$output       .= '<span class="post-updated" itemprop="dateModified"> ' . $modified_on . '</span>';
	$output       .= '</span>';
	return $output;
}

add_filter( 'astra_post_date', 'your_prefix_post_date' );