%order_date in shipping notification is always 1970-01-01 00:00

When generating the shipping notification the variable %order_date is always replaced with 1970-01-01 00:00.

I have corrected it in my installation by changing the following line in function shipping_preview_email($txnid, $preview = null)

from:
"%order_date" => format_date($t['created']),

to:
"%order_date" => format_date($t->created),