diff --git a/swh/core/json.py b/swh/core/json.py
new file mode 100644
index 0000000..59902e5
--- /dev/null
+++ b/swh/core/json.py
@@ -0,0 +1,52 @@
+# Copyright (C) 2015  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+import base64
+import datetime
+from json import JSONDecoder, JSONEncoder
+
+import dateutil.parser
+
+
+class SWHJSONEncoder(JSONEncoder):
+    def default(self, o):
+        if isinstance(o, bytes):
+            return {
+                'swhtype': 'bytes',
+                'd': base64.b85encode(o).decode('ascii'),
+            }
+        elif isinstance(o, datetime.datetime):
+            return {
+                'swhtype': 'datetime',
+                'd': o.isoformat(),
+            }
+        try:
+            return super().default(o)
+        except TypeError as e:
+            try:
+                iterable = iter(o)
+            except TypeError:
+                raise e from None
+            else:
+                return list(iterable)
+
+
+class SWHJSONDecoder(JSONDecoder):
+    def decode_data(self, o):
+        if isinstance(o, dict):
+            datatype = o.get('swhtype')
+            if datatype == 'bytes':
+                return base64.b85decode(o['d'])
+            elif datatype == 'datetime':
+                return dateutil.parser.parse(o['d'])
+            return {key: self.decode_data(value) for key, value in o.items()}
+        if isinstance(o, list):
+            return [self.decode_data(value) for value in o]
+        else:
+            return o
+
+    def raw_decode(self, s, idx=0):
+        data, index = super().raw_decode(s, idx)
+        return self.decode_data(data), index
diff --git a/swh/core/tests/test_json.py b/swh/core/tests/test_json.py
new file mode 100644
index 0000000..d42b30f
--- /dev/null
+++ b/swh/core/tests/test_json.py
@@ -0,0 +1,46 @@
+# Copyright (C) 2015  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+import datetime
+import json
+import unittest
+
+from nose.tools import istest
+
+from swh.core.json import SWHJSONDecoder, SWHJSONEncoder
+
+
+class JSON(unittest.TestCase):
+    def setUp(self):
+        self.tz = datetime.timezone(datetime.timedelta(minutes=118))
+
+        self.data = {
+            "bytes": b"123456789\x99\xaf\xff\x00\x12",
+            "datetime_naive": datetime.datetime(2015, 1, 1, 12, 4, 42, 231455),
+            "datetime_tz": datetime.datetime(2015, 3, 4, 18, 25, 13, 1234,
+                                             tzinfo=self.tz),
+            "datetime_utc": datetime.datetime(2015, 3, 4, 18, 25, 13, 1234,
+                                              tzinfo=datetime.timezone.utc),
+        }
+
+        self.encoded_data = {
+            "bytes": {"swhtype": "bytes", "d": "F)}kWH8wXmIhn8j01^"},
+            "datetime_naive": {"swhtype": "datetime",
+                               "d": "2015-01-01T12:04:42.231455"},
+            "datetime_tz": {"swhtype": "datetime",
+                            "d": "2015-03-04T18:25:13.001234+01:58"},
+            "datetime_utc": {"swhtype": "datetime",
+                             "d": "2015-03-04T18:25:13.001234+00:00"},
+        }
+
+    @istest
+    def round_trip(self):
+        data = json.dumps(self.data, cls=SWHJSONEncoder)
+        self.assertEqual(self.data, json.loads(data, cls=SWHJSONDecoder))
+
+    @istest
+    def encode(self):
+        data = json.dumps(self.data, cls=SWHJSONEncoder)
+        self.assertEqual(self.encoded_data, json.loads(data))