diff --git a/swh/deposit/tests/api/test_checks.py b/swh/deposit/tests/api/test_checks.py
index 0eeb75ee..9896db60 100644
--- a/swh/deposit/tests/api/test_checks.py
+++ b/swh/deposit/tests/api/test_checks.py
@@ -1,539 +1,573 @@
 # Copyright (C) 2017-2022  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
 
 # disable flake8 on this file because of line length
 # flake8: noqa
 
 import re
 import textwrap
 from typing import Any, Dict
 from xml.etree import ElementTree
 
 import pytest
 
 from swh.deposit.api.checks import (
     METADATA_PROVENANCE_KEY,
     SUGGESTED_FIELDS_MISSING,
     check_metadata,
 )
 
 METADATA_PROVENANCE_DICT: Dict[str, Any] = {
     "swh:deposit": {
         METADATA_PROVENANCE_KEY: {"schema:url": "some-metadata-provenance-url"}
     }
 }
 
 XMLNS = """xmlns="http://www.w3.org/2005/Atom"
                    xmlns:swh="https://www.softwareheritage.org/schema/2018/deposit"
                    xmlns:codemeta="https://doi.org/10.5063/SCHEMA/CODEMETA-2.0"
                    xmlns:schema="http://schema.org/" """
 
 PROVENANCE_XML = """
                 <swh:deposit>
                     <swh:metadata-provenance>
                         <schema:url>some-metadata-provenance-url</schema:url>
                     </swh:metadata-provenance>
                 </swh:deposit>"""
 
 _parameters1 = [
     textwrap.dedent(metadata_ok)
     for (metadata_ok,) in [
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <name>foo</name>
                 <author>someone</author>
                 {PROVENANCE_XML}
             </entry>
             """,
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <name>foo</name>
                 <author>no one</author>
                 {PROVENANCE_XML}
             </entry>
             """,
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>some url</url>
                 <codemeta:name>bar</codemeta:name>
                 <codemeta:author>no one</codemeta:author>
                 {PROVENANCE_XML}
             </entry>
             """,
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>some url</url>
                 <external_identifier>some id</external_identifier>
                 <name>nar</name>
                 <author>no one</author>
                 <codemeta:datePublished>2020-12-21</codemeta:datePublished>
                 <codemeta:dateCreated>2020-12-21</codemeta:dateCreated>
+                <codemeta:dateModified>2020-12-25</codemeta:dateModified>
+                {PROVENANCE_XML}
+            </entry>
+            """,
+        ),
+        (
+            # technically, only Date is allowed for datePublished; but we allow DateTime
+            # for backward compatibility with old swh-deposit versions
+            f"""\
+            <entry {XMLNS}>
+                <url>some url</url>
+                <external_identifier>some id</external_identifier>
+                <name>nar</name>
+                <author>no one</author>
+                <codemeta:datePublished>2020-12-21T12:00:00</codemeta:datePublished>
+                <codemeta:dateCreated>2020-12-21T12:00:00</codemeta:dateCreated>
+                <codemeta:dateModified>2020-12-25T12:00:00</codemeta:dateModified>
                 {PROVENANCE_XML}
             </entry>
             """,
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
             </entry>
             """,
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:add_to_origin>
                         <swh:origin url="http://example.org" />
                     </swh:add_to_origin>
                     <swh:metadata-provenance>
                         <schema:url>some-metadata-provenance-url</schema:url>
                     </swh:metadata-provenance>
                 </swh:deposit>
             </entry>
             """,
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:reference>
                         <swh:origin url="http://example.org" />
                     </swh:reference>
                     <swh:metadata-provenance>
                         <schema:url>some-metadata-provenance-url</schema:url>
                     </swh:metadata-provenance>
                 </swh:deposit>
             </entry>
             """,
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:reference>
                         <swh:object swhid="swh:1:dir:0000000000000000000000000000000000000000" />
                     </swh:reference>
                     <swh:metadata-provenance>
                         <schema:url>some-metadata-provenance-url</schema:url>
                     </swh:metadata-provenance>
                 </swh:deposit>
             </entry>
             """,
         ),
     ]
 ]
 
 
 @pytest.mark.parametrize(
     "metadata_ok", _parameters1,
 )
 def test_api_checks_check_metadata_ok(metadata_ok, swh_checks_deposit):
     actual_check, detail = check_metadata(ElementTree.fromstring(metadata_ok))
     assert actual_check is True, f"Unexpected result: {detail}"
     if "swh:deposit" in metadata_ok:
         # no missing suggested field
         assert detail is None
     else:
         # missing suggested field
         assert detail == {
             "metadata": [
                 {
                     "fields": [METADATA_PROVENANCE_KEY],
                     "summary": SUGGESTED_FIELDS_MISSING,
                 }
             ]
         }
 
 
 _parameters2 = [
     (textwrap.dedent(metadata_ko), expected_summary)
     for (metadata_ko, expected_summary) in [
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <author>someone</author>
                 {PROVENANCE_XML}
             </entry>
             """,
             {
                 "summary": "Mandatory fields are missing",
                 "fields": ["atom:name or atom:title or codemeta:name"],
             },
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>foobar</title>
                 {PROVENANCE_XML}
             </entry>
             """,
             {
                 "summary": "Mandatory fields are missing",
                 "fields": ["atom:author or codemeta:author"],
             },
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <codemeta:title>bar</codemeta:title>
                 <author>someone</author>
                 {PROVENANCE_XML}
             </entry>
             """,
             {
                 "summary": "Mandatory fields are missing",
                 "fields": ["atom:name or atom:title or codemeta:name"],
             },
         ),
         (
             f"""\
             <entry xmlns:atom="http://www.w3.org/2005/Atom"
                    xmlns:swh="https://www.softwareheritage.org/schema/2018/deposit"
                    xmlns:codemeta="https://doi.org/10.5063/SCHEMA/CODEMETA-2.0"
                    xmlns:schema="http://schema.org/">
                 <atom:url>something</atom:url>
                 <atom:external_identifier>something-else</atom:external_identifier>
                 <atom:title>foobar</atom:title>
                 <author>foo</author>
                 {PROVENANCE_XML}
             </entry>
             """,
             {
                 "summary": "Mandatory fields are missing",
                 "fields": ["atom:author or codemeta:author"],
             },
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <authorblahblah>someone</authorblahblah>
                 {PROVENANCE_XML}
             </entry>
             """,
             {
                 "summary": "Mandatory fields are missing",
                 "fields": ["atom:author or codemeta:author"],
             },
         ),
     ]
 ]
 
 
 @pytest.mark.parametrize("metadata_ko,expected_summary", _parameters2)
 def test_api_checks_check_metadata_ko(
     metadata_ko, expected_summary, swh_checks_deposit
 ):
     actual_check, error_detail = check_metadata(ElementTree.fromstring(metadata_ko))
     assert actual_check is False
     assert error_detail == {"metadata": [expected_summary]}
 
 
 _parameters3 = [
     (textwrap.dedent(metadata_ko), expected_summary)
     for (metadata_ko, expected_summary) in [
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <codemeta:datePublished>2020-aa-21</codemeta:datePublished>
                 <codemeta:dateCreated>2020-12-bb</codemeta:dateCreated>
                 {PROVENANCE_XML}
             </entry>
             """,
             [
                 {
                     "summary": ".*Reason: invalid value '2020-aa-21'.*",
                     "fields": ["codemeta:datePublished"],
                 },
                 {
                     "summary": ".*Reason: invalid value '2020-12-bb'.*",
                     "fields": ["codemeta:dateCreated"],
                 },
             ],
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>some url</url>
                 <external_identifier>someid</external_identifier>
                 <title>bar</title>
                 <author>no one</author>
                 <codemeta:datePublished>2020-aa-21</codemeta:datePublished>
                 <codemeta:dateCreated>2020-12-bb</codemeta:dateCreated>
             </entry>
             """,
             [
                 {
                     "summary": ".*Reason: invalid value '2020-aa-21'.*",
                     "fields": ["codemeta:datePublished"],
                 },
                 {
                     "summary": ".*Reason: invalid value '2020-12-bb'.*",
                     "fields": ["codemeta:dateCreated"],
                 },
             ],
         ),
+        (
+            f"""\
+            <entry {XMLNS}>
+                <url>some url</url>
+                <external_identifier>someid</external_identifier>
+                <title>bar</title>
+                <author>no one</author>
+                <codemeta:dateModified>2020-12-aa</codemeta:dateModified>
+            </entry>
+            """,
+            [
+                {
+                    "summary": ".*Reason: invalid value '2020-12-aa'.*",
+                    "fields": ["codemeta:dateModified"],
+                },
+            ],
+        ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:invalid>
                         <swh:origin url="http://example.org" />
                     </swh:invalid>
                 </swh:deposit>
             </entry>
             """,
             [
                 {
                     "summary": (
                         r".*Reason: Unexpected child with tag 'swh:invalid'.*"
                         r"Instance:.*swh:invalid.*"
                     ),
                     "fields": ["swh:deposit"],
                 }
             ],
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:add_to_origin>
                         <swh:origin url="http://example.org" />
                     </swh:add_to_origin>
                     <swh:add_to_origin>
                         <swh:origin url="http://example.org" />
                     </swh:add_to_origin>
                 </swh:deposit>
             </entry>
             """,
             [
                 {
                     "summary": (
                         r".*Reason: Unexpected child with tag 'swh:add_to_origin'.*"
                     ),
                     "fields": ["swh:deposit"],
                 }
             ],
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:add_to_origin>
                         <swh:origin url="http://example.org" />
                     </swh:add_to_origin>
                     <swh:create_origin>
                         <swh:origin url="http://example.org" />
                     </swh:create_origin>
                 </swh:deposit>
             </entry>
             """,
             [
                 {
                     "summary": (
                         r".*Reason: assertion test if false.*"
                         r"Schema:\n*"
                         r' *<xsd:assert[^>]+ id="swhdeposit-incompatible-create-and-add".*'
                     ),
                     "fields": ["swh:deposit"],
                 }
             ],
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:create_origin>
                         <swh:origin url="http://example.org" />
                     </swh:create_origin>
                     <swh:reference>
                         <swh:origin url="http://example.org" />
                     </swh:reference>
                 </swh:deposit>
             </entry>
             """,
             [
                 {
                     "summary": (
                         r".*Reason: assertion test if false.*"
                         r"Schema:\n*"
                         r' *<xsd:assert[^>]+ id="swhdeposit-incompatible-create-and-reference".*'
                     ),
                     "fields": ["swh:deposit"],
                 }
             ],
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:add_to_origin>
                         <swh:origin url="http://example.org" />
                     </swh:add_to_origin>
                     <swh:reference>
                         <swh:origin url="http://example.org" />
                     </swh:reference>
                 </swh:deposit>
             </entry>
             """,
             [
                 {
                     "summary": (
                         r".*Reason: assertion test if false.*"
                         r"Schema:\n*"
                         r' *<xsd:assert[^>]+ id="swhdeposit-incompatible-add-and-reference".*'
                     ),
                     "fields": ["swh:deposit"],
                 }
             ],
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:reference>
                         <swh:object swhid="swh:1:dir:0000000000000000000000000000000000000000" />
                         <swh:origin url="http://example.org" />
                     </swh:reference>
                 </swh:deposit>
             </entry>
             """,
             [
                 {
                     "summary": r".*Reason: Unexpected child with tag 'swh:origin'.*",
                     "fields": ["swh:deposit"],
                 },
             ],
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:reference>
                         <swh:origin url="http://example.org" />
                         <swh:origin url="http://example.org" />
                     </swh:reference>
                 </swh:deposit>
             </entry>
             """,
             [
                 {
                     "summary": r".*Reason: Unexpected child with tag 'swh:origin'.*",
                     "fields": ["swh:deposit"],
                 },
             ],
         ),
         (
             f"""\
             <entry {XMLNS}>
                 <url>something</url>
                 <external_identifier>something-else</external_identifier>
                 <title>bar</title>
                 <author>someone</author>
                 <swh:deposit>
                     <swh:reference>
                         <swh:origin url="http://example.org" />
                         <swh:object swhid="swh:1:dir:0000000000000000000000000000000000000000" />
                     </swh:reference>
                 </swh:deposit>
             </entry>
             """,
             [
                 {
                     "summary": r".*Reason: Unexpected child with tag 'swh:object'.*",
                     "fields": ["swh:deposit"],
                 },
             ],
         ),
     ]
 ]
 
 
 @pytest.mark.parametrize("metadata_ko,expected_summaries", _parameters3)
 def test_api_checks_check_metadata_ko_schema(
     metadata_ko, expected_summaries, swh_checks_deposit
 ):
     actual_check, error_detail = check_metadata(ElementTree.fromstring(metadata_ko))
     assert actual_check is False
     assert len(error_detail["metadata"]) == len(expected_summaries), error_detail[
         "metadata"
     ]
 
     for (detail, expected_summary) in zip(error_detail["metadata"], expected_summaries):
         assert detail["fields"] == expected_summary["fields"]
 
         # xmlschema returns very detailed errors, we cannot reasonably test them
         # for equality
         summary = detail["summary"]
         assert re.match(
             expected_summary["summary"], summary, re.DOTALL
         ), f"Failed to match {expected_summary['summary']!r} with:\n{summary}"
diff --git a/swh/deposit/xsd/codemeta.xsd b/swh/deposit/xsd/codemeta.xsd
index 6b117726..5544172a 100644
--- a/swh/deposit/xsd/codemeta.xsd
+++ b/swh/deposit/xsd/codemeta.xsd
@@ -1,16 +1,19 @@
 <?xml version="1.0" encoding="utf-8"?>
 <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
             targetNamespace="https://doi.org/10.5063/SCHEMA/CODEMETA-2.0"
             xmlns:codemeta="https://doi.org/10.5063/SCHEMA/CODEMETA-2.0"
             elementFormDefault="qualified"
             >
   <!-- This is a non-exhaustive schema used to find likely errors in XML-encoded
        Codemeta documents -->
 
+  <!-- technically, only Date is allowed for datePublished; but we allow DateTime
+       for backward compatibility with old swh-deposit versions -->
   <xsd:element name="datePublished" type="codemeta:DateOrDateTime" />
   <xsd:element name="dateCreated" type="codemeta:DateOrDateTime" />
+  <xsd:element name="dateModified" type="codemeta:DateOrDateTime" />
 
   <xsd:simpleType name="DateOrDateTime">
     <xsd:union memberTypes="xsd:date xsd:dateTime"/>
   </xsd:simpleType>
 </xsd:schema>