diff --git a/swh/loader/package/deposit/loader.py b/swh/loader/package/deposit/loader.py
--- a/swh/loader/package/deposit/loader.py
+++ b/swh/loader/package/deposit/loader.py
@@ -86,7 +86,6 @@
                 RawExtrinsicMetadataCore(
                     format="sword-v2-atom-codemeta-v2-in-json",
                     metadata=raw_metadata_from_origin,
-                    discovery_date=None,
                 ),
             ],
         )
@@ -185,7 +184,6 @@
             RawExtrinsicMetadataCore(
                 format="sword-v2-atom-codemeta-v2-in-json",
                 metadata=json.dumps(origin_metadata["metadata"]).encode(),
-                discovery_date=None,
             )
         ]
 
diff --git a/swh/loader/package/loader.py b/swh/loader/package/loader.py
--- a/swh/loader/package/loader.py
+++ b/swh/loader/package/loader.py
@@ -64,7 +64,7 @@
 
     format = attr.ib(type=str)
     metadata = attr.ib(type=bytes)
-    discovery_date = attr.ib(type=Optional[datetime.datetime])
+    discovery_date = attr.ib(type=Optional[datetime.datetime], default=None)
     """Defaults to the visit date."""
 
 
diff --git a/swh/loader/package/nixguix/loader.py b/swh/loader/package/nixguix/loader.py
--- a/swh/loader/package/nixguix/loader.py
+++ b/swh/loader/package/nixguix/loader.py
@@ -89,9 +89,7 @@
     def get_extrinsic_snapshot_metadata(self):
         return [
             RawExtrinsicMetadataCore(
-                format="nixguix-sources-json",
-                metadata=self.raw_sources,
-                discovery_date=None,
+                format="nixguix-sources-json", metadata=self.raw_sources,
             ),
         ]
 
diff --git a/swh/loader/package/npm/loader.py b/swh/loader/package/npm/loader.py
--- a/swh/loader/package/npm/loader.py
+++ b/swh/loader/package/npm/loader.py
@@ -120,9 +120,7 @@
     def get_extrinsic_snapshot_metadata(self):
         return [
             RawExtrinsicMetadataCore(
-                format="replicate-npm-package-json",
-                metadata=self._raw_info,
-                discovery_date=None,
+                format="replicate-npm-package-json", metadata=self._raw_info,
             ),
         ]