Calendula Flower Harvesting

I am looking for information in the proper time to harvest Calendula flower to be dried. Should they be harvested before, during, after full bloom or once the seeds have set? Should just the flower be harvested or should the stem be taken as well?

Only the flowers are harvested. They are picked just after they open, dried and stripped of their petals. The petals are used fresh or dried in infusions, liquid extracts, tinctures and culinary uses. Macerated in oil they are used externally.