Showing new posts count since last login

Getting the posts after a specific time can be achieved with 2 steps.

You need to store the last login time of the user.
Changing the query to pull the posts which are modified after the above login time.

The below function will store the last login time of the user.

// Associating a function to login hook
add_action ( 'wp_login', 'set_last_login' );

function set_last_login ( $login ) {
    $user = get_userdatabylogin ( $login );

    // Setting the last login of the user
    update_usermeta ( $user->ID, 'last_login', date ( 'Y-m-d H:i:s' ) );
}

Then you need to collect the last login time of the logged in user and modify the query as below. Currently it shows a list of posts but you should easily be able to modify this to output a number by counting the number of times it goes through the loop.

<?php
// Get current user object
$current_user = wp_get_current_user();

// Get the last login time of the user
$last_login_time = get_user_meta ( $current_user->ID, 'last_login', true );

// WP_Query with post modified time
$the_query = new WP_Query(
                array(
                    'date_query' =>
                        array(
                            'column' => 'post_modified',
                            'after' => $last_login_time,
                        )
                )
            );
?>
<?php if ( $the_query->have_posts() ) : ?>

    <?php // Start the Loop ?>
    <?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
        <?php // Show the output ?>
    <?php endwhile; ?>

<?php endif; ?>
<?php
// Restore original Post Data
wp_reset_postdata();
?>

This is awesome! thanks!