Keith Wood’s jQuery script didn’t specify a way to include milliseconds. You’re welcome to suggest it to him via his webpage (bottom of the page).
Static countdowns on page reload are not available in this plug-in (yet), but in the meantime you can use PHP in a page template to generate and hard-code the expiration date based on the page load time. Something like:
$add_time = 300; // 5 minutes = 300 seconds
$target_timestamp = time() + $add_time;
$target_time = date( 'H:i:s', $target_timestamp );
$target_date = date( 'm/d/Y', $target_timestamp );
echo do_shortcode( '[countdown date="' . $target_date . '" time="' . $target_time . '"]' );
I didn’t test this, so be careful if you copy-paste.
Thanks for the suggestion, I’ll add it to the list for the next version 🙂