Defining Where Excerpt Begins

I am trying to shave off a few characters or pieces of text we have in our posts. We have the date and source from which the story in our posts cam but I would not like that included in our excerpt and the formatting people insist that it must remain at the top of the post. How would I go about specifying exactly where I would like the excerpt to begin in the post? Could I have it begin at something like a tag or could I set the number of characters to skip before it begins? Any help would be greatly appreciated. Here is my code thus far:

<phpcode>
<?php $my_query = new WP_Query('category_name=science&showposts=5'); ?>
<?php while ($my_query->have_posts()) : $my_query->the_post(); ?>
<div id="postlist_container">
<h4 class="und"></h4>
<?php get_the_image(array( 'image_scan' => true , 'image_class' => 'small_image_left','width' => 80 , 'height' => 80)); ?><div class="post_desc"><date><?php the_time('M j, Y') ?></date> &middot; <a href="<?php the_permalink() ?>">
<?php the_title(); ?></a> <br /><br /><?php the_excerpt_max_charlength(250); ?>
</div>
</div>
<div class="clear"></div>
<?php endwhile; ?>
<?php

function the_excerpt_max_charlength($charlength) {
    $excerpt = get_the_excerpt();
    $charlength++;

    if ( mb_strlen( $excerpt ) > $charlength ) {
        $subex = mb_substr( $excerpt, 0, $charlength - 5 );
        $exwords = explode( ' ', $subex );
        $excut = - ( mb_strlen( $exwords[ count( $exwords ) - 1 ] ) );
        if ( $excut < 0 ) {
            echo mb_substr( $subex, 0, $excut );
        } else {
            echo $subex;
        }
        echo '[...]';
    } else {
        echo $excerpt;
    }
}
?>

</phpcode>