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 +107,7 @@
 
     return TimestampWithTimezone(
         timestamp=Timestamp(
-            seconds=int(date.timestamp()), microseconds=date.microsecond,
+            seconds=int(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
@@ -72,6 +72,78 @@
                 "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):