Date of last modification if at least one day later than the post creation date.

wp_last_post_change.php

<?php
/**
 * Date of last modification if at least one day later than the post creation date.
 *
 * @author Thomas Scholz
 * @param string $template Parsed with sprintf(). Must contain a '%s'.
 * @param string|int $time_diff Time difference as string for strtotime() or integer as seconds
 * @return string|void
 */
function t5_post_last_mode( $template = '%s', $time_diff )
{
	if ( empty( $time_diff ) )
		$time_diff = 86400; // 86400 seconds == 1 day
		
	// UINX time stamps in seconds:
	$changed = get_post_modified_time( 'U' );
	
	if ( is_string( $time_diff ) )
	{
		$made = strtotime( $time_diff, get_the_date( 'U' ) );
		$delta = 0;
	} else
	{
		$made = get_the_date( 'U' );
		$delta = &$time_diff;
	} 

	if ( ( $changed - $made ) > $delta )
	{
		// readable formatted date
		$human_last_mod = get_post_modified_time( 
			get_option( 'date_format' ),
			FALSE, // GMT
			NULL,  // Post
			TRUE   // translate
		);
		return sprintf( $template, $human_last_mod );
	}
}

print t5_post_last_mode( 'Last modified: %s', '2 hour' );
print t5_post_last_mode( 'Last modified: %s', 86400 );