diff --git a/swh/model/identifiers.py b/swh/model/identifiers.py
--- a/swh/model/identifiers.py
+++ b/swh/model/identifiers.py
@@ -688,7 +688,7 @@
     ```
     target_type: $ValueOfMetadataTargetType
     target: $UrlOrSwhid
-    discovery_date: $ISO8601
+    discovery_date: $Timestamp
     authority: $StrWithoutSpaces $IRI
     fetcher: $Str $Version
     format: $StrWithoutSpaces
@@ -712,6 +712,12 @@
 
     $Swhid are core SWHIDs, as defined in :ref:`persistent-identifiers`.
 
+    $Timestamp is a decimal representation of the integer number of seconds since
+    the UNIX epoch (1970-01-01 00:00:00 UTC), with no leading '0'
+    (unless the timestamp value is zero) and no timezone.
+    It may be negative by prefixing it with a '-', which must not be followed
+    by a '0'.
+
     Newlines in $Bytes, $Str, and $Iri are escaped as with other git fields,
     ie. by adding a space after them.
 
@@ -719,10 +725,16 @@
       str: the intrinsic identifier for `metadata`
 
     """
+    discovery_date = metadata["discovery_date"]
+    if discovery_date.microsecond != 0:
+        raise ValueError(f"discovery_date={discovery_date} has microsecond != 0")
+    timestamp = discovery_date.timestamp()
+    assert timestamp.is_integer()
+
     headers = [
         (b"target_type", metadata["type"].encode("ascii")),
         (b"target", str(metadata["target"]).encode()),
-        (b"discovery_date", metadata["discovery_date"].isoformat().encode("ascii")),
+        (b"discovery_date", str(int(timestamp)).encode("ascii")),
         (
             b"authority",
             f"{metadata['authority']['type']} {metadata['authority']['url']}".encode(),
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
@@ -803,10 +803,10 @@
 
     def test_minimal(self):
         manifest = (
-            b"raw_extrinsic_metadata 245\0"
+            b"raw_extrinsic_metadata 230\0"
             b"target_type content\n"
             b"target swh:1:cnt:568aaf43d83b2c3df8067f3bedbb97d83260be6d\n"
-            b"discovery_date 2021-01-25T11:27:51+00:00\n"
+            b"discovery_date 1611574071\n"
             b"authority forge https://forge.softwareheritage.org/\n"
             b"fetcher swh-phabricator-metadata-fetcher 0.0.1\n"
             b"format json\n"
@@ -820,15 +820,15 @@
         )
         self.assertEqual(
             identifiers.raw_extrinsic_metadata_identifier(self.minimal),
-            "da734f1531f830b7282ee01c5e0c0dfe7ecc99e9",
+            "e35827936a4ae7c351a92eda0eeb36da07da315f",
         )
 
     def test_maximal(self):
         manifest = (
-            b"raw_extrinsic_metadata 568\0"
+            b"raw_extrinsic_metadata 553\0"
             b"target_type content\n"
             b"target swh:1:cnt:568aaf43d83b2c3df8067f3bedbb97d83260be6d\n"
-            b"discovery_date 2021-01-25T11:27:51+00:00\n"
+            b"discovery_date 1611574071\n"
             b"authority forge https://forge.softwareheritage.org/\n"
             b"fetcher swh-phabricator-metadata-fetcher 0.0.1\n"
             b"format json\n"
@@ -849,7 +849,7 @@
         )
         self.assertEqual(
             identifiers.raw_extrinsic_metadata_identifier(self.maximal),
-            "0d9e3bb9a72850e32bfb575f612cfad1a7e6b66a",
+            "7523fa6cef72ced2935242a45def7c5a36d0f609",
         )
 
     def test_nonascii_path(self):
@@ -858,10 +858,10 @@
             "path": b"/ab\nc/d\xf0\x9f\xa4\xb7e\x00f",
         }
         manifest = (
-            b"raw_extrinsic_metadata 266\0"
+            b"raw_extrinsic_metadata 251\0"
             b"target_type content\n"
             b"target swh:1:cnt:568aaf43d83b2c3df8067f3bedbb97d83260be6d\n"
-            b"discovery_date 2021-01-25T11:27:51+00:00\n"
+            b"discovery_date 1611574071\n"
             b"authority forge https://forge.softwareheritage.org/\n"
             b"fetcher swh-phabricator-metadata-fetcher 0.0.1\n"
             b"format json\n"
@@ -877,7 +877,27 @@
         )
         self.assertEqual(
             identifiers.raw_extrinsic_metadata_identifier(metadata),
-            "63f4cb28396e00926ab7ebfd96e96b60227fc11a",
+            "caacc32905ebf4659a633a2dc856ccd6412991dc",
+        )
+
+    def test_timezone_insensitive(self):
+        """Checks the timezone of the datetime.datetime does not affect the
+        hashed manifest."""
+        utc_plus_one = datetime.timezone(datetime.timedelta(hours=1))
+        metadata = {
+            **self.minimal,
+            "discovery_date": datetime.datetime(
+                2021, 1, 25, 12, 27, 51, tzinfo=utc_plus_one,
+            ),
+        }
+
+        self.assertEqual(
+            identifiers.raw_extrinsic_metadata_identifier(self.minimal),
+            identifiers.raw_extrinsic_metadata_identifier(metadata),
+        )
+        self.assertEqual(
+            identifiers.raw_extrinsic_metadata_identifier(metadata),
+            "e35827936a4ae7c351a92eda0eeb36da07da315f",
         )