diff --git a/xee-interpreter/src/atomic/cast_datetime.rs b/xee-interpreter/src/atomic/cast_datetime.rs
index 362b378a6..b9e957c63 100644
--- a/xee-interpreter/src/atomic/cast_datetime.rs
+++ b/xee-interpreter/src/atomic/cast_datetime.rs
@@ -62,7 +62,8 @@ impl atomic::Atomic {
         let mut s = String::new();
         let offset = date_time.offset;
         let date_time = date_time.date_time;
-        s.push_str(&date_time.format("%Y-%m-%dT%H:%M:%S").to_string());
+        Self::push_canonical_year(&mut s, date_time.year());
+        s.push_str(&date_time.format("-%m-%dT%H:%M:%S").to_string());
         let millis = date_time.and_utc().timestamp_subsec_millis();
         Self::push_millis(&mut s, millis);
         if let Some(offset) = offset {
@@ -75,7 +76,8 @@ impl atomic::Atomic {
         date_time: &chrono::DateTime<chrono::FixedOffset>,
     ) -> String {
         let mut s = String::new();
-        s.push_str(&date_time.format("%Y-%m-%dT%H:%M:%S").to_string());
+        Self::push_canonical_year(&mut s, date_time.year());
+        s.push_str(&date_time.format("-%m-%dT%H:%M:%S").to_string());
         let millis = date_time.timestamp_subsec_millis();
         Self::push_millis(&mut s, millis);
         let offset = date_time.offset();
@@ -100,7 +102,8 @@ impl atomic::Atomic {
         let mut s = String::new();
         let offset = date.offset;
         let date = date.date;
-        s.push_str(&date.format("%Y-%m-%d").to_string());
+        Self::push_canonical_year(&mut s, date.year());
+        s.push_str(&date.format("-%m-%d").to_string());
         if let Some(offset) = offset {
             Self::push_canonical_time_zone_offset(&mut s, &offset);
         }
@@ -206,6 +209,16 @@ impl atomic::Atomic {
         }
     }
 
+    fn push_canonical_year(s: &mut String, year: i32) {
+        // chrono's %Y writes years above 9999 with a leading '+', which is
+        // not part of the XSD lexical space, so write the year ourselves
+        if year >= 0 {
+            s.push_str(&format!("{:04}", year));
+        } else {
+            s.push_str(&format!("-{:04}", year.abs()));
+        }
+    }
+
     fn push_canonical_time_zone_offset(s: &mut String, offset: &chrono::FixedOffset) {
         let seconds = offset.local_minus_utc();
         if seconds == 0 {
diff --git a/xee-xpath/tests/canonical_years.rs b/xee-xpath/tests/canonical_years.rs
new file mode 100644
index 000000000..480416468
--- /dev/null
+++ b/xee-xpath/tests/canonical_years.rs
@@ -0,0 +1,58 @@
+// Canonical serialization of years outside 1..=9999.
+//
+// chrono's %Y formats years above 9999 with a leading '+', which is not
+// valid in XSD lexical space, so the engine's own output failed to cast
+// back: xs:dateTime(string(xs:dateTime("10000-01-01T00:00:00Z"))) raised
+// FORG0001.
+
+use xee_xpath::{error, Documents, Queries, Query};
+
+fn eval(xpath: &str) -> error::Result<String> {
+    let mut documents = Documents::new();
+    let queries = Queries::default();
+    let q = queries.one(xpath, |_, item| Ok(item.try_into_value::<String>()?))?;
+    q.execute_build_context(&mut documents, |_| {})
+}
+
+#[test]
+fn test_five_digit_years_serialize_without_plus() {
+    assert_eq!(
+        eval(r#"string(xs:dateTime("10000-01-01T00:00:00Z"))"#).unwrap(),
+        "10000-01-01T00:00:00Z"
+    );
+    assert_eq!(
+        eval(r#"string(xs:dateTimeStamp("10000-01-01T00:00:00Z"))"#).unwrap(),
+        "10000-01-01T00:00:00Z"
+    );
+    assert_eq!(
+        eval(r#"string(xs:date("10000-01-01"))"#).unwrap(),
+        "10000-01-01"
+    );
+}
+
+#[test]
+fn test_five_digit_year_round_trips_through_the_engine() {
+    assert_eq!(
+        eval(r#"string(xs:dateTime(string(xs:dateTime("10000-01-01T00:00:00Z"))))"#).unwrap(),
+        "10000-01-01T00:00:00Z"
+    );
+    assert_eq!(
+        eval(r#"string(xs:date(string(xs:date("10000-01-01"))))"#).unwrap(),
+        "10000-01-01"
+    );
+}
+
+#[test]
+fn test_other_year_forms_stay_canonical() {
+    // negative years and year zero were already canonical
+    assert_eq!(
+        eval(r#"string(xs:dateTime("-0005-03-01T12:00:00"))"#).unwrap(),
+        "-0005-03-01T12:00:00"
+    );
+    assert_eq!(
+        eval(r#"string(xs:date("0000-01-01"))"#).unwrap(),
+        "0000-01-01"
+    );
+    // the gregorian types formatted years manually all along
+    assert_eq!(eval(r#"string(xs:gYear("10000"))"#).unwrap(), "10000");
+}