Effective dating: links that stop being true — Turn it on per relation field

Dating is a setting on a Relation field, in the field editor. With it on, each link can carry a from and a to — this employee reported to that manager from January to June; this product sat in that category until the range was renamed. Switching it on changes nothing until somebody actually dates something. An undated link is true at every instant, so existing data keeps behaving exactly as it did.