diff --git a/lib/app/v3/champion/replica.dart b/lib/app/v3/champion/replica.dart
index 0e7ece48f..6f0f0dda2 100644
--- a/lib/app/v3/champion/replica.dart
+++ b/lib/app/v3/champion/replica.dart
@@ -25,6 +25,20 @@ class Replica {
     // interprets. The desktop CLI is what acts on it.
     "recur",
   ];
+
+  /// Format a map value for the FFI string map.
+  ///
+  /// Dart's [DateTime.toString] uses a space (`2026-10-03 12:00:00.000Z`),
+  /// which Rust `parse_datetime` rejects. RFC3339 via [DateTime.toIso8601String]
+  /// keeps due/wait/start intact on create.
+  @visibleForTesting
+  static String attributeValueToString(dynamic value) {
+    if (value is DateTime) {
+      return value.toUtc().toIso8601String();
+    }
+    return value.toString();
+  }
+
   /// Create a task, returning null on success or the reason it was refused.
   ///
   /// This used to answer with a success/failure sentinel that no caller read —
@@ -46,7 +60,7 @@ class Replica {
       if (newTask.containsKey(attr) &&
           newTask[attr] != null &&
           attrs.contains(attr)) {
-        map[attr] = newTask[attr].toString();
+        map[attr] = attributeValueToString(newTask[attr]);
       }
     }
     map['uuid'] = UuidV4().generate();
diff --git a/rust/src/api.rs b/rust/src/api.rs
index f16fcc1ba..3d76d470a 100644
--- a/rust/src/api.rs
+++ b/rust/src/api.rs
@@ -11,10 +11,21 @@ use crate::storage::open_replica;
 use crate::utils::error::TcHelperError;
 
 fn parse_datetime(input: &str) -> Option<DateTime<Utc>> {
-    if input.trim().is_empty() {
+    let input = input.trim();
+    if input.is_empty() {
         return None;
     }
-    input.parse::<DateTime<Utc>>().ok()
+    if let Ok(dt) = input.parse::<DateTime<Utc>>() {
+        return Some(dt);
+    }
+    // Dart's DateTime.toString() emits a space instead of 'T'
+    // (`2026-10-03 12:00:00.000Z`). That is not RFC3339, so accept it as a
+    // fallback — otherwise set_due/set_wait silently clear the field.
+    if let Some((date, rest)) = input.split_once(' ') {
+        format!("{date}T{rest}").parse::<DateTime<Utc>>().ok()
+    } else {
+        None
+    }
 }
 
 /// Return every task in the replica as a JSON array string.
@@ -1503,6 +1514,42 @@ mod add_task_attribute_tests {
         assert_eq!(field(&path, &uuid, "recur").as_deref(), Some("monthly"));
         fs::remove_dir_all(&tmp).ok();
     }
+
+    #[test]
+    fn dart_datetime_tostring_due_and_wait_are_stored() {
+        // Regression for #662: Dart DateTime.toString() uses a space, not 'T'.
+        // parse_datetime used to return None and set_due/set_wait cleared the
+        // field while add_task still reported success.
+        let (tmp, path) = dir();
+        let (uuid, res) = add(
+            &path,
+            &[
+                ("due", "2026-10-03 12:00:00.000Z"),
+                ("wait", "2026-10-04 08:30:00.000Z"),
+            ],
+        );
+        res.expect("add_task");
+
+        let json = get_all_tasks_json(path.clone()).expect("get_all_tasks_json");
+        let tasks: Vec<Value> = serde_json::from_str(&json).expect("parse json");
+        let task = tasks
+            .into_iter()
+            .find(|t| t.get("uuid").and_then(|u| u.as_str()) == Some(uuid.as_str()))
+            .expect("task not found");
+
+        // Serializer emits due/wait as epoch-second numbers.
+        let due = task
+            .get("due")
+            .and_then(|v| v.as_i64().or_else(|| v.as_str().and_then(|s| s.parse().ok())))
+            .expect("due must be stored");
+        let wait = task
+            .get("wait")
+            .and_then(|v| v.as_i64().or_else(|| v.as_str().and_then(|s| s.parse().ok())))
+            .expect("wait must be stored");
+        assert_eq!(due, 1_791_028_800); // 2026-10-03T12:00:00Z
+        assert_eq!(wait, 1_791_102_600); // 2026-10-04T08:30:00Z
+        fs::remove_dir_all(&tmp).ok();
+    }
 }
 
 #[cfg(test)]
diff --git a/test/v3/champion/replica_attribute_value_test.dart b/test/v3/champion/replica_attribute_value_test.dart
new file mode 100644
index 000000000..59d9f6bf7
--- /dev/null
+++ b/test/v3/champion/replica_attribute_value_test.dart
@@ -0,0 +1,37 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:taskwarrior/app/v3/champion/replica.dart';
+
+void main() {
+  group('Replica.attributeValueToString', () {
+    test('formats UTC DateTime as RFC3339 with T separator', () {
+      final due = DateTime.utc(2026, 10, 3, 12, 0, 0);
+      final encoded = Replica.attributeValueToString(due);
+
+      expect(encoded, '2026-10-03T12:00:00.000Z');
+      expect(encoded.contains(' '), isFalse);
+      expect(DateTime.parse(encoded), due);
+    });
+
+    test('normalizes local DateTime to UTC before encoding', () {
+      final local = DateTime(2026, 10, 3, 17, 30);
+      final encoded = Replica.attributeValueToString(local);
+
+      expect(encoded, local.toUtc().toIso8601String());
+      expect(encoded.contains('T'), isTrue);
+    });
+
+    test('stringifies non-DateTime values with toString', () {
+      expect(Replica.attributeValueToString('buy milk'), 'buy milk');
+      expect(Replica.attributeValueToString('H'), 'H');
+      expect(Replica.attributeValueToString(42), '42');
+    });
+
+    test('Dart DateTime.toString is not accepted as-is by RFC3339 parsers', () {
+      // Documents the bug this helper fixes: space-separated form fails
+      // chrono's DateTime<Utc> parse used by the Rust FFI.
+      final due = DateTime.utc(2026, 10, 3, 12, 0, 0);
+      expect(due.toString(), '2026-10-03 12:00:00.000Z');
+      expect(Replica.attributeValueToString(due), isNot(due.toString()));
+    });
+  });
+}