This page was last modified on 1 January 1970
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!