echo php variable on external javascript possible?

Yes. Make the javascript variable global and you can access it inside you external js file. Something like this

<script type="text/javascript">
  var dateFuture = new Date(<?php echo $var1; ?>);
</script>
<script src="your-external-js-file.js" type="text/javascript"></script>

In your-external-js-file.js, you can access the dateFuture.

Or you can encapsulate the code in external js file in a class and pass on the date from php as a parameter to the constructor of that class.

The external JavaScript file itself can point to a PHP file — provided that the PHP file outputs valid JavaScript. That way, you can do something like the following:

myJS.php:

<?php

// Initialize your PHP variable(s) here, or include the PHP script(s) to do so.
$var1="2011,9,31,18,0,0";

...

?>

dateFuture = new Date(<?php echo $var1; ?>);

In your HTML file:

<script type="text/javascript" src="myJS.php"></script>

Since the output of myJS.php is purely JavaScript code, the file extension will not matter to the browser. Same way as your PHP code currently outputs purely HTML code, and your browser understands how to parse that as well.

If your purpose is to move the javascript code to an external script for better modularization, you can move it to a php file and then reference php file as javascript.

<script type="text/javascript" src="myscript.php"></script>

Inside myscript.php -

<?
Header("content-type: text/javascript");
dateFuture = new Date(<?php echo $var1; ?>);
?>

Whilst the answers given by Jonathan Newmuis and RonakG are perfectly acceptable and will work, the purpose of this answer is to answer your question as close to the setup you've got now as possible. However I'd personally agree with RonakG's answer.

If you're using Apache on your server then add the following line to your .htaccess file: AddType application/x-httpd-php .js. Alternatively you could add that code into the Apache configuration if performance is an concern for you.

The goal of that code is, essentially, to say that PHP should parse all files ending in ".js" as if they were ".php"