diff --git a/lib/src/core/definitions/thing_description.dart b/lib/src/core/definitions/thing_description.dart
index fd786014..b88553ea 100644
--- a/lib/src/core/definitions/thing_description.dart
+++ b/lib/src/core/definitions/thing_description.dart
@@ -178,12 +178,14 @@ class ThingDescription {
       result["version"] = version.toJson();
     }
 
+    final created = this.created;
     if (created != null) {
-      result["created"] = created.toString();
+      result["created"] = created.toIso8601String();
     }
 
+    final modified = this.modified;
     if (modified != null) {
-      result["modified"] = modified.toString();
+      result["modified"] = modified.toIso8601String();
     }
 
     if (support != null) {
diff --git a/test/core/definitions/serialization_test.dart b/test/core/definitions/serialization_test.dart
index 3917e980..0fa7cdae 100644
--- a/test/core/definitions/serialization_test.dart
+++ b/test/core/definitions/serialization_test.dart
@@ -23,8 +23,10 @@ void main() {
         "version": {
           "instance": "1.0.0",
         },
-        "created": "2024-05-25 00:00:00.000",
-        "modified": "2024-05-25 00:00:00.000",
+        // TODO: Should fields like these be able to be "roundtripped"?
+        //       I.e., getting the same result back that was put in?
+        "created": "2024-05-25T00:00:00.000",
+        "modified": "2024-05-25T00:00:00.000",
         "support": "https://example.org",
         "base": "https://example.org",
         "id": "urn:uuid:5edfed77-fc4e-46d4-a550-ef7f07592fbd",