diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -112,7 +112,6 @@
         """Serializes the date as a string and omits the visit id if it is
         `None`."""
         ov = super().to_dict()
-        ov['date'] = str(self.date)
         if ov['visit'] is None:
             del ov['visit']
         return ov
@@ -120,9 +119,13 @@
     @classmethod
     def from_dict(cls, d):
         """Parses the date from a string, and accepts missing visit ids."""
+        d = d.copy()
+        date = d.pop('date')
         return cls(
-            origin=Origin.from_dict(d['origin']),
-            date=dateutil.parser.parse(d['date']),
+            origin=Origin.from_dict(d.pop('origin')),
+            date=(date
+                  if isinstance(date, datetime.datetime)
+                  else dateutil.parser.parse(date)),
             visit=d.get('visit'))
 
 
diff --git a/swh/model/tests/test_hypothesis_strategies.py b/swh/model/tests/test_hypothesis_strategies.py
--- a/swh/model/tests/test_hypothesis_strategies.py
+++ b/swh/model/tests/test_hypothesis_strategies.py
@@ -3,6 +3,8 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import datetime
+
 import attr
 from hypothesis import given
 
@@ -30,7 +32,8 @@
     elif isinstance(obj, list):
         for value in obj:
             assert_nested_dict(value)
-    elif isinstance(obj, (int, float, str, bytes, bool, type(None))):
+    elif isinstance(obj, (int, float, str, bytes, bool, type(None),
+                          datetime.datetime)):
         pass
     else:
         assert False, obj