diff --git a/requirements.txt b/requirements.txt
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,7 @@
 # Add here external Python modules dependencies, one per line. Module names
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
-attrs >= 21.1.1
+attrs != 21.1.0  # https://github.com/python-attrs/attrs/issues/804
 attrs_strict >= 0.0.7
 deprecated
 hypothesis
diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -378,7 +378,7 @@
             raise ValueError("Microseconds must be in [0, 1000000[.")
 
 
-@attr.s(frozen=True, slots=True, init=False)
+@attr.s(frozen=True, slots=True)
 class TimestampWithTimezone(BaseModel):
     """Represents a TZ-aware timestamp from a VCS."""
 
@@ -386,9 +386,6 @@
 
     timestamp = attr.ib(type=Timestamp, validator=type_validator())
 
-    offset = attr.ib(type=int, validator=type_validator())
-    negative_utc = attr.ib(type=bool, validator=type_validator())
-
     offset_bytes = attr.ib(type=bytes, validator=type_validator())
     """Raw git representation of the timezone, as an offset from UTC.
     It should follow this format: ``+HHMM`` or ``-HHMM`` (including ``+0000`` and
@@ -398,123 +395,6 @@
     original objects, so it may differ from this format when they do.
     """
 
-    def __init__(
-        self,
-        timestamp: Timestamp,
-        offset: int = None,
-        negative_utc: bool = None,
-        offset_bytes: bytes = None,
-    ):
-        if offset_bytes is None:
-            if offset is None:
-                raise AttributeError("Neither 'offset' nor 'offset_bytes' was passed.")
-            if negative_utc is None:
-                raise AttributeError(
-                    "Neither 'negative_utc' nor 'offset_bytes' was passed."
-                )
-            negative = offset < 0 or negative_utc
-            (hours, minutes) = divmod(abs(offset), 60)
-            offset_bytes = f"{'-' if negative else '+'}{hours:02}{minutes:02}".encode()
-        else:
-            offset = self._parse_offset_bytes(offset_bytes)
-            negative_utc = offset == 0 and offset_bytes.startswith(b"-")
-
-        self.__attrs_init__(  # type: ignore
-            timestamp=timestamp,
-            offset=offset,
-            negative_utc=negative_utc,
-            offset_bytes=offset_bytes,
-        )
-
-    @offset.validator
-    def check_offset(self, attribute, value):
-        """Checks the offset is a 16-bits signed integer (in theory, it
-        should always be between -14 and +14 hours)."""
-        if not (-(2 ** 15) <= value < 2 ** 15):
-            # max 14 hours offset in theory, but you never know what
-            # you'll find in the wild...
-            raise ValueError("offset too large: %d minutes" % value)
-
-        self._check_offsets_match()
-
-    @negative_utc.validator
-    def check_negative_utc(self, attribute, value):
-        if self.offset and value:
-            raise ValueError("negative_utc can only be True is offset=0")
-
-        self._check_offsets_match()
-
-    @offset_bytes.validator
-    def check_offset_bytes(self, attribute, value):
-        if not set(value) <= _OFFSET_CHARS:
-            raise ValueError(f"invalid characters in offset_bytes: {value!r}")
-
-        self._check_offsets_match()
-
-    @staticmethod
-    def _parse_offset_bytes(offset_bytes: bytes) -> int:
-        """Parses an ``offset_bytes`` value (in Git's ``[+-]HHMM`` format),
-        and returns the corresponding numeric values (in number of minutes).
-
-        Tries to account for some mistakes in the format, to support incorrect
-        Git implementations.
-
-        >>> TimestampWithTimezone._parse_offset_bytes(b"+0000")
-        0
-        >>> TimestampWithTimezone._parse_offset_bytes(b"-0000")
-        0
-        >>> TimestampWithTimezone._parse_offset_bytes(b"+0200")
-        120
-        >>> TimestampWithTimezone._parse_offset_bytes(b"-0200")
-        -120
-        >>> TimestampWithTimezone._parse_offset_bytes(b"+200")
-        120
-        >>> TimestampWithTimezone._parse_offset_bytes(b"-200")
-        -120
-        >>> TimestampWithTimezone._parse_offset_bytes(b"+02")
-        120
-        >>> TimestampWithTimezone._parse_offset_bytes(b"-02")
-        -120
-        >>> TimestampWithTimezone._parse_offset_bytes(b"+0010")
-        10
-        >>> TimestampWithTimezone._parse_offset_bytes(b"-0010")
-        -10
-        >>> TimestampWithTimezone._parse_offset_bytes(b"+200000000000000000")
-        0
-        >>> TimestampWithTimezone._parse_offset_bytes(b"+0160")  # 60 minutes...
-        0
-        """
-        offset_str = offset_bytes.decode()
-        assert offset_str[0] in "+-"
-        sign = int(offset_str[0] + "1")
-        if len(offset_str) <= 3:
-            hours = int(offset_str[1:])
-            minutes = 0
-        else:
-            hours = int(offset_str[1:-2])
-            minutes = int(offset_str[-2:])
-
-        offset = sign * (hours * 60 + minutes)
-        if (0 <= minutes <= 59) and (-(2 ** 15) <= offset < 2 ** 15):
-            return offset
-        else:
-            # can't parse it to a reasonable value; give up and pretend it's UTC.
-            return 0
-
-    def _check_offsets_match(self):
-        offset = self._parse_offset_bytes(self.offset_bytes)
-        if offset != self.offset:
-            raise ValueError(
-                f"offset_bytes ({self.offset_bytes!r}) does not match offset "
-                f"{divmod(self.offset, 60)}"
-            )
-
-        if offset == 0 and self.negative_utc != self.offset_bytes.startswith(b"-"):
-            raise ValueError(
-                f"offset_bytes ({self.offset_bytes!r}) does not match negative_utc "
-                f"({self.negative_utc})"
-            )
-
     @classmethod
     def from_numeric_offset(
         cls, timestamp: Timestamp, offset: int, negative_utc: bool
@@ -525,12 +405,7 @@
         negative = offset < 0 or negative_utc
         (hours, minutes) = divmod(abs(offset), 60)
         offset_bytes = f"{'-' if negative else '+'}{hours:02}{minutes:02}".encode()
-        tstz = TimestampWithTimezone(
-            timestamp=timestamp,
-            offset_bytes=offset_bytes,
-            offset=offset,
-            negative_utc=negative_utc,
-        )
+        tstz = TimestampWithTimezone(timestamp=timestamp, offset_bytes=offset_bytes)
         assert tstz.offset == offset, (tstz.offset, offset)
         return tstz
 
@@ -559,7 +434,7 @@
             timestamp = Timestamp(seconds=seconds, microseconds=microseconds)
 
             if "offset_bytes" in time_representation:
-                return TimestampWithTimezone(
+                return cls(
                     timestamp=timestamp,
                     offset_bytes=time_representation["offset_bytes"],
                 )
@@ -593,7 +468,7 @@
             # TODO: warn when using from_dict() on an int
             seconds = time_representation
             timestamp = Timestamp(seconds=time_representation, microseconds=0)
-            return TimestampWithTimezone(timestamp=timestamp, offset_bytes=b"+0000")
+            return cls(timestamp=timestamp, offset_bytes=b"+0000")
         else:
             raise ValueError(
                 f"TimestampWithTimezone.from_dict received non-integer timestamp: "
@@ -625,9 +500,59 @@
         tstz = cls.from_datetime(dt)
         if dt.tzname() == "-00:00":
             assert tstz.offset_bytes == b"+0000"
-            tstz = attr.evolve(tstz, offset_bytes=b"-0000", negative_utc=True)
+            tstz = attr.evolve(tstz, offset_bytes=b"-0000")
         return tstz
 
+    @staticmethod
+    def _parse_offset_bytes(offset_bytes: bytes) -> int:
+        """Parses an ``offset_bytes`` value (in Git's ``[+-]HHMM`` format),
+        and returns the corresponding numeric values (in number of minutes).
+
+        Tries to account for some mistakes in the format, to support incorrect
+        Git implementations.
+
+        >>> TimestampWithTimezone._parse_offset_bytes(b"+0000")
+        0
+        >>> TimestampWithTimezone._parse_offset_bytes(b"-0000")
+        0
+        >>> TimestampWithTimezone._parse_offset_bytes(b"+0200")
+        120
+        >>> TimestampWithTimezone._parse_offset_bytes(b"-0200")
+        -120
+        >>> TimestampWithTimezone._parse_offset_bytes(b"+200")
+        120
+        >>> TimestampWithTimezone._parse_offset_bytes(b"-200")
+        -120
+        >>> TimestampWithTimezone._parse_offset_bytes(b"+02")
+        120
+        >>> TimestampWithTimezone._parse_offset_bytes(b"-02")
+        -120
+        >>> TimestampWithTimezone._parse_offset_bytes(b"+0010")
+        10
+        >>> TimestampWithTimezone._parse_offset_bytes(b"-0010")
+        -10
+        >>> TimestampWithTimezone._parse_offset_bytes(b"+200000000000000000")
+        0
+        >>> TimestampWithTimezone._parse_offset_bytes(b"+0160")  # 60 minutes...
+        0
+        """
+        offset_str = offset_bytes.decode()
+        assert offset_str[0] in "+-"
+        sign = int(offset_str[0] + "1")
+        if len(offset_str) <= 3:
+            hours = int(offset_str[1:])
+            minutes = 0
+        else:
+            hours = int(offset_str[1:-2])
+            minutes = int(offset_str[-2:])
+
+        offset = sign * (hours * 60 + minutes)
+        if (0 <= minutes <= 59) and (-(2 ** 15) <= offset < 2 ** 15):
+            return offset
+        else:
+            # can't parse it to a reasonable value; give up and pretend it's UTC.
+            return 0
+
     def offset_minutes(self):
         """Returns the offset, as a number of minutes since UTC.
 
@@ -648,7 +573,12 @@
         ... ).offset_minutes()
         330
         """
-        return self.offset
+        return self._parse_offset_bytes(self.offset_bytes)
+
+    @property
+    def offset(self):
+        """Deprecated alias of :meth:`offset_minutes`."""
+        return self.offset_minutes()
 
 
 @attr.s(frozen=True, slots=True)
diff --git a/swh/model/tests/test_identifiers.py b/swh/model/tests/test_identifiers.py
--- a/swh/model/tests/test_identifiers.py
+++ b/swh/model/tests/test_identifiers.py
@@ -21,7 +21,6 @@
     Release,
     Revision,
     Snapshot,
-    Timestamp,
     TimestampWithTimezone,
 )
 
@@ -1036,104 +1035,51 @@
     # with current input dict format (offset_bytes)
     (
         {"timestamp": 12345, "offset_bytes": b"+0000"},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0000",},
     ),
     (
         {"timestamp": 12345, "offset_bytes": b"-0000"},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"-0000",
-            "offset": 0,
-            "negative_utc": True,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"-0000",},
     ),
     (
         {"timestamp": 12345, "offset_bytes": b"+0200"},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0200",
-            "offset": 120,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0200",},
     ),
     (
         {"timestamp": 12345, "offset_bytes": b"-0200"},
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"-0200",},
+    ),
+    (
+        {"timestamp": 12345, "offset_bytes": b"--700"},
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"--700",},
+    ),
+    (
+        {"timestamp": 12345, "offset_bytes": b"1234567"},
         {
             "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"-0200",
-            "offset": -120,
-            "negative_utc": False,
+            "offset_bytes": b"1234567",
         },
     ),
-    # not working yet:
-    # (
-    #     {"timestamp": 12345, "offset_bytes": b"--700"},
-    #     {
-    #         "timestamp": {"seconds": 12345, "microseconds": 0},
-    #         "offset_bytes": b"--700",
-    #         "offset": 0,
-    #         "negative_utc": False,
-    #     },
-    # ),
-    # (
-    #     {"timestamp": 12345, "offset_bytes": b"1234567"},
-    #     {
-    #         "timestamp": {"seconds": 12345, "microseconds": 0},
-    #         "offset_bytes": b"1234567",
-    #         "offset": 0,
-    #         "negative_utc": False,
-    #     },
-    # ),
     # with old-style input dicts (numeric offset + optional negative_utc):
     (
         {"timestamp": 12345, "offset": 0},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0000",},
     ),
     (
         {"timestamp": 12345, "offset": 0, "negative_utc": False},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0000",},
     ),
     (
         {"timestamp": 12345, "offset": 0, "negative_utc": False},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0000",},
     ),
     (
         {"timestamp": 12345, "offset": 0, "negative_utc": None},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0000",},
     ),
     (
         {"timestamp": {"seconds": 12345}, "offset": 0, "negative_utc": None},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0000",},
     ),
     (
         {
@@ -1141,12 +1087,7 @@
             "offset": 0,
             "negative_utc": None,
         },
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0000",},
     ),
     (
         {
@@ -1157,27 +1098,15 @@
         {
             "timestamp": {"seconds": 12345, "microseconds": 100},
             "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
         },
     ),
     (
         {"timestamp": 12345, "offset": 0, "negative_utc": True},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"-0000",
-            "offset": 0,
-            "negative_utc": True,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"-0000",},
     ),
     (
         {"timestamp": 12345, "offset": 0, "negative_utc": None},
-        {
-            "timestamp": {"seconds": 12345, "microseconds": 0},
-            "offset_bytes": b"+0000",
-            "offset": 0,
-            "negative_utc": False,
-        },
+        {"timestamp": {"seconds": 12345, "microseconds": 0}, "offset_bytes": b"+0000",},
     ),
 ]
 
@@ -1187,35 +1116,6 @@
     assert TimestampWithTimezone.from_dict(dict_input).to_dict() == expected
 
 
-def test_timestampwithtimezone_init():
-    ts = Timestamp(seconds=1234567, microseconds=0)
-    tstz = TimestampWithTimezone(
-        timestamp=ts, offset=120, negative_utc=False, offset_bytes=b"+0200"
-    )
-    assert tstz.timestamp == ts
-    assert tstz.offset == 120
-    assert tstz.negative_utc is False
-    assert tstz.offset_bytes == b"+0200"
-
-    assert tstz == TimestampWithTimezone(timestamp=ts, offset=120, negative_utc=False)
-    assert tstz == TimestampWithTimezone(timestamp=ts, offset_bytes=b"+0200")
-
-    assert tstz != TimestampWithTimezone(timestamp=ts, offset_bytes=b"+0100")
-
-    tstz = TimestampWithTimezone(
-        timestamp=ts, offset=0, negative_utc=True, offset_bytes=b"-0000"
-    )
-    assert tstz.timestamp == ts
-    assert tstz.offset == 0
-    assert tstz.negative_utc is True
-    assert tstz.offset_bytes == b"-0000"
-
-    assert tstz == TimestampWithTimezone(timestamp=ts, offset=0, negative_utc=True)
-    assert tstz == TimestampWithTimezone(timestamp=ts, offset_bytes=b"-0000")
-
-    assert tstz != TimestampWithTimezone(timestamp=ts, offset_bytes=b"+0000")
-
-
 TS_DICTS_INVALID_TIMESTAMP = [
     {"timestamp": 1.2, "offset": 0},
     {"timestamp": "1", "offset": 0},
@@ -1262,8 +1162,6 @@
     assert TimestampWithTimezone.from_dict(date).to_dict() == {
         "timestamp": {"seconds": seconds, "microseconds": microsecond},
         "offset_bytes": offset_bytes,
-        "offset": offset,
-        "negative_utc": False,
     }
 
 
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
@@ -498,7 +498,7 @@
     with pytest.raises(AttributeTypeError):
         TimestampWithTimezone(timestamp=datetime.datetime.now(), offset_bytes=b"+0000")
 
-    with pytest.raises((AttributeTypeError, AttributeError, TypeError)):
+    with pytest.raises((AttributeTypeError, TypeError)):
         TimestampWithTimezone(timestamp=ts, offset_bytes=0)