diff --git a/swh/deposit/tests/api/test_checks.py b/swh/deposit/tests/api/test_checks.py
--- a/swh/deposit/tests/api/test_checks.py
+++ b/swh/deposit/tests/api/test_checks.py
@@ -81,6 +81,23 @@
                 <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>
             """,
@@ -314,6 +331,23 @@
                 },
             ],
         ),
+        (
+            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}>
diff --git a/swh/deposit/xsd/codemeta.xsd b/swh/deposit/xsd/codemeta.xsd
--- a/swh/deposit/xsd/codemeta.xsd
+++ b/swh/deposit/xsd/codemeta.xsd
@@ -7,8 +7,11 @@
   <!-- 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 historical reasons -->
   <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"/>