diff --git a/swh/storage/postgresql/converters.py b/swh/storage/postgresql/converters.py
--- a/swh/storage/postgresql/converters.py
+++ b/swh/storage/postgresql/converters.py
@@ -1,9 +1,10 @@
-# Copyright (C) 2015-2020  The Software Heritage developers
+# Copyright (C) 2015-2021  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 math
 from typing import Any, Dict, Optional
 import warnings
 
@@ -106,7 +107,9 @@
 
     return TimestampWithTimezone(
         timestamp=Timestamp(
-            seconds=int(date.timestamp()), microseconds=date.microsecond,
+            # we use floor() instead of int() to round down, because of negative dates
+            seconds=math.floor(date.timestamp()),
+            microseconds=date.microsecond,
         ),
         offset=offset,
         negative_utc=neg_utc_offset,
diff --git a/swh/storage/tests/test_postgresql_converters.py b/swh/storage/tests/test_postgresql_converters.py
--- a/swh/storage/tests/test_postgresql_converters.py
+++ b/swh/storage/tests/test_postgresql_converters.py
@@ -20,57 +20,144 @@
 from swh.storage.postgresql import converters
 
 
-def test_date_to_db():
-    date_to_db = converters.date_to_db
-    assert date_to_db(None) == {"timestamp": None, "offset": 0, "neg_utc_offset": None}
-
-    assert date_to_db(
-        TimestampWithTimezone(
-            timestamp=Timestamp(seconds=1234567890, microseconds=0,),
-            offset=120,
-            negative_utc=False,
-        )
-    ) == {
-        "timestamp": "2009-02-13T23:31:30+00:00",
-        "offset": 120,
-        "neg_utc_offset": False,
-    }
-
-    assert date_to_db(
-        TimestampWithTimezone(
-            timestamp=Timestamp(seconds=1123456789, microseconds=0,),
-            offset=0,
-            negative_utc=True,
-        )
-    ) == {
-        "timestamp": "2005-08-07T23:19:49+00:00",
-        "offset": 0,
-        "neg_utc_offset": True,
-    }
-
-    assert date_to_db(
-        TimestampWithTimezone(
-            timestamp=Timestamp(seconds=1234567890, microseconds=0,),
-            offset=42,
-            negative_utc=False,
-        )
-    ) == {
-        "timestamp": "2009-02-13T23:31:30+00:00",
-        "offset": 42,
-        "neg_utc_offset": False,
-    }
-
-    assert date_to_db(
-        TimestampWithTimezone(
-            timestamp=Timestamp(seconds=1634366813, microseconds=0,),
-            offset=-120,
-            negative_utc=False,
+@pytest.mark.parametrize(
+    "model_date,db_date",
+    [
+        (None, {"timestamp": None, "offset": 0, "neg_utc_offset": None}),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=1234567890, microseconds=0,),
+                offset=120,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "2009-02-13T23:31:30+00:00",
+                "offset": 120,
+                "neg_utc_offset": False,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=1123456789, microseconds=0,),
+                offset=0,
+                negative_utc=True,
+            ),
+            {
+                "timestamp": "2005-08-07T23:19:49+00:00",
+                "offset": 0,
+                "neg_utc_offset": True,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=1234567890, microseconds=0,),
+                offset=42,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "2009-02-13T23:31:30+00:00",
+                "offset": 42,
+                "neg_utc_offset": False,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=1634366813, microseconds=0,),
+                offset=-120,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "2021-10-16T06:46:53+00:00",
+                "offset": -120,
+                "neg_utc_offset": False,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=0, microseconds=0,),
+                offset=-120,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "1970-01-01T00:00:00+00:00",
+                "offset": -120,
+                "neg_utc_offset": False,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=0, microseconds=1,),
+                offset=-120,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "1970-01-01T00:00:00.000001+00:00",
+                "offset": -120,
+                "neg_utc_offset": False,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=-1, microseconds=0,),
+                offset=-120,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "1969-12-31T23:59:59+00:00",
+                "offset": -120,
+                "neg_utc_offset": False,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=-1, microseconds=1,),
+                offset=-120,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "1969-12-31T23:59:59.000001+00:00",
+                "offset": -120,
+                "neg_utc_offset": False,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=-3600, microseconds=0,),
+                offset=-120,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "1969-12-31T23:00:00+00:00",
+                "offset": -120,
+                "neg_utc_offset": False,
+            },
+        ),
+        (
+            TimestampWithTimezone(
+                timestamp=Timestamp(seconds=-3600, microseconds=1,),
+                offset=-120,
+                negative_utc=False,
+            ),
+            {
+                "timestamp": "1969-12-31T23:00:00.000001+00:00",
+                "offset": -120,
+                "neg_utc_offset": False,
+            },
+        ),
+    ],
+)
+def test_date(model_date, db_date):
+    assert converters.date_to_db(model_date) == db_date
+    assert (
+        converters.db_to_date(
+            date=None
+            if db_date["timestamp"] is None
+            else datetime.datetime.fromisoformat(db_date["timestamp"]),
+            offset=db_date["offset"],
+            neg_utc_offset=db_date["neg_utc_offset"],
         )
-    ) == {
-        "timestamp": "2021-10-16T06:46:53+00:00",
-        "offset": -120,
-        "neg_utc_offset": False,
-    }
+        == model_date
+    )
 
 
 def test_db_to_author():