Heuristic: when only one border known set other to ±½a

datings.py

@@ -15,6 +15,7 @@
 
 logger = logging.getLogger(__name__)
 
+HALF_INTERVAL_CORRECTION = datetime.timedelta(365/2)
 
 def parse_datestr(datestr: str) -> datetime.date:
     if datestr is None:
@@ -188,8 +189,13 @@ def add_to_graph(self, G: nx.MultiDiGraph):
             for source in self.sources:
                 if self.start is not None:
                     G.add_edge(self.date_before, item, kind=self.start_attr[0], source=source, dating=self, xml=self.xmlsource)
+                    if self.end is None:
+                        G.add_edge(item, self.date_before + HALF_INTERVAL_CORRECTION, kind='not_after', source=BiblSource('faust://heuristic'), xml=self.xmlsource)
                 if self.end is not None:
                     G.add_edge(item, self.date_after, kind=self.end_attr[0], source=source, dating=self, xml=self.xmlsource)
+                    if self.start is None:
+                        G.add_edge(self.date_after - HALF_INTERVAL_CORRECTION, item, kind='not_before', source=BiblSource('faust://heuristic'), xml=self.xmlsource)
+
 
 
 class RelativeDating(_AbstractDating):