Uses the get_the_modified_time function to created a shortcode, just like the [entry-published] shortcode in Hybrid Core

[entry-modified] shortcode

add_shortcode( 'entry-modified', 'krogs_entry_modified_shortcode' );

function krogs_entry_modified_shortcode( $attr ) {

	$domain = hybrid_get_textdomain();
	$attr = shortcode_atts( array( 'before' => '', 'after' => '', 'format' => get_option( 'date_format' ) ), $attr );

	$modified = '<abbr class="modified" title="' . sprintf( get_the_modified_time( esc_attr__( 'l, F jS, Y, g:i a', $domain ) ) ) . '">' . sprintf( get_the_modified_time( $attr['format'] ) ) . '</abbr>';
	return $attr['before'] . $modified . $attr['after'];
	
}