diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -19,6 +19,7 @@
 import datetime
 from enum import Enum
 import hashlib
+import math
 from typing import Any, Dict, Iterable, Optional, Tuple, TypeVar, Union
 
 import attr
@@ -477,7 +478,7 @@
             microseconds = time_representation.microsecond
             if microseconds:
                 time_representation = time_representation.replace(microsecond=0)
-            seconds = int(time_representation.timestamp())
+            seconds = math.floor(time_representation.timestamp())
             utcoffset = time_representation.utcoffset()
             if utcoffset is None:
                 raise ValueError(
diff --git a/swh/model/tests/test_model.py b/swh/model/tests/test_model.py
--- a/swh/model/tests/test_model.py
+++ b/swh/model/tests/test_model.py
@@ -500,17 +500,45 @@
 
 
 def test_timestampwithtimezone_from_datetime():
+    # Typical case
     tz = datetime.timezone(datetime.timedelta(minutes=+60))
     date = datetime.datetime(2020, 2, 27, 14, 39, 19, tzinfo=tz)
-
     tstz = TimestampWithTimezone.from_datetime(date)
-
     assert tstz == TimestampWithTimezone(
         timestamp=Timestamp(seconds=1582810759, microseconds=0,),
         offset=60,
         negative_utc=False,
     )
 
+    # Typical case (close to epoch)
+    tz = datetime.timezone(datetime.timedelta(minutes=+60))
+    date = datetime.datetime(1970, 1, 1, 1, 0, 5, tzinfo=tz)
+    tstz = TimestampWithTimezone.from_datetime(date)
+    assert tstz == TimestampWithTimezone(
+        timestamp=Timestamp(seconds=5, microseconds=0,), offset=60, negative_utc=False,
+    )
+
+    # non-integer number of seconds before UNIX epoch
+    date = datetime.datetime(
+        1969, 12, 31, 23, 59, 59, 100000, tzinfo=datetime.timezone.utc
+    )
+    tstz = TimestampWithTimezone.from_datetime(date)
+    assert tstz == TimestampWithTimezone(
+        timestamp=Timestamp(seconds=-1, microseconds=100000,),
+        offset=0,
+        negative_utc=False,
+    )
+
+    # timezone offset with non-integer number of seconds, for dates before epoch
+    # we round down to the previous second, so it should be the same as
+    # 1969-01-01T23:59:59Z
+    tz = datetime.timezone(datetime.timedelta(microseconds=100000))
+    date = datetime.datetime(1970, 1, 1, 0, 0, 0, tzinfo=tz)
+    tstz = TimestampWithTimezone.from_datetime(date)
+    assert tstz == TimestampWithTimezone(
+        timestamp=Timestamp(seconds=-1, microseconds=0,), offset=0, negative_utc=False,
+    )
+
 
 def test_timestampwithtimezone_from_naive_datetime():
     date = datetime.datetime(2020, 2, 27, 14, 39, 19)