This page was last modified on 1 January 1970 Including the following code:
&lt;?php print 'This page was last modified on ' . date('j F Y', $node-&gt;changed);?&gt;

in node.tpl.php displays the correct information on the site. However, I would like to display it in the footer instead at the end of the content so have moved it to page.tpl.php.
The problem with this is that on the homepage the following is being displayed:

This page was last modified on 1 January 1970

Which is clearly incorrect.
In addition, These error messages are displayed on every page:

Notice: Undefined variable: node in include() (line 201 of /home/mysite/public_html/sites/all/themes/mysite/templates/page.tpl.php).
Notice: Trying to get property of non-object in include() (line 201 of /home/mysite/public_html/sites/all/themes/mysite/templates/page.tpl.php).

How do I include the Page last updated text in the footer of every page?
You are trying to access the property of a loaded Node's object in the page template. This variable, $node, will only be available to the page template, typically page.tpl.php, only if the current path is loading a node.

In other words, if you are loading a path like node/NODE_ID, then you are guaranteed the variable will be available. Otherwise it won't be.

The error you are seeing is because of this reason. Try to navigate to a valid node URL and the error will be gone, the variable will be defined, and you can access it normally.

Provide a conditional check in your page template to make sure the variable exists before you access it.

Good Luck!