Replace YYYY in the copyright line by the current year

Follow up for #5004

Author: amenk

app/code/Magento/Theme/Block/Html/Footer.php

@@ -93,7 +93,7 @@ public function getCopyright()
                 \Magento\Store\Model\ScopeInterface::SCOPE_STORE
             );
         }
-        return __($this->_copyright);
+        return $this->replaceCurrentYear(__($this->_copyright));
     }
 
     /**
@@ -133,4 +133,12 @@ protected function getCacheLifetime()
     {
         return parent::getCacheLifetime() ?: 3600;
     }
+    
+    /**
+     * Replace YYYY by the current year
+     */
+    private function replaceCurrentYear($text)
+    {
+        return str_replace('YYYY', (new \DateTime())->format('Y'), $text)   
+    }
 }