diff --git a/swh/provenance/provenance.py b/swh/provenance/provenance.py
--- a/swh/provenance/provenance.py
+++ b/swh/provenance/provenance.py
@@ -227,12 +227,12 @@
 
 class IsochroneNode:
     def __init__(
-        self, entry: DirectoryEntry, dates: Dict[bytes, datetime] = {}, depth: int = 0
+        self, entry: DirectoryEntry, date: Optional[datetime] = None, depth: int = 0
     ):
         self.entry = entry
         self.depth = depth
-        self.date = dates.get(self.entry.id, None)
-        self.known = self.date is not None
+        self.date = date
+        self.known: bool = self.date is not None
         self.children: List[IsochroneNode] = []
         self.maxdate: Optional[datetime] = None
 
@@ -240,7 +240,8 @@
         self, child: DirectoryEntry, dates: Dict[bytes, datetime] = {}
     ) -> "IsochroneNode":
         assert isinstance(self.entry, DirectoryEntry) and self.date is None
-        node = IsochroneNode(child, dates=dates, depth=self.depth + 1)
+        node = IsochroneNode(
+            child, date=dates.get(child.id, None), depth=self.depth + 1)
         self.children.append(node)
         return node
 
@@ -255,9 +256,8 @@
     assert revision.root == directory.id
 
     # Build the nodes structure
-    root = IsochroneNode(directory)
-    root.date = provenance.directory_get_date_in_isochrone_frontier(directory)
-    root.known = root.date is not None
+    root_date = provenance.directory_get_date_in_isochrone_frontier(directory)
+    root = IsochroneNode(directory, date=root_date)
     stack = [root]
     logging.debug(
         f"Recursively creating graph for revision {hash_to_hex(revision.id)}..."