diff --git a/integration/testdata/set_create_operation/tests.test.ts b/integration/testdata/set_create_operation/tests.test.ts
index cd76d1b08..efed0985e 100644
--- a/integration/testdata/set_create_operation/tests.test.ts
+++ b/integration/testdata/set_create_operation/tests.test.ts
@@ -1,6 +1,6 @@
 import { actions, resetDatabase, models } from "@teamkeel/testing";
 import { test, expect, beforeEach } from "vitest";
-import { ThingType } from "@teamkeel/sdk";
+import { ThingType, Duration } from "@teamkeel/sdk";
 
 beforeEach(resetDatabase);
 
@@ -160,14 +160,14 @@ test("duration set attribute on required field - set to P1D - is P1D", async ()
 
 test("duration set attribute from explicit input - set to P2D - is P2D", async () => {
   const thing = await actions.createDurationFromExplicitInput({
-    explDuration: "P2D",
+    explDuration: Duration.fromISOString("P2D")
   });
   expect(thing.requiredDuration).toEqual("P2D");
 });
 
 test("duration set attribute from implicit input - set to P2D - is P2D", async () => {
   const thing = await actions.createDurationFromImplicitInput({
-    requiredDuration: "P2D",
+    requiredDuration: Duration.fromISOString("P2D"),
   });
   expect(thing.optionalDuration).toEqual("P2D");
   expect(thing.requiredDuration).toEqual("P2D");
diff --git a/packages/testing-runtime/src/Executor.mjs b/packages/testing-runtime/src/Executor.mjs
index 4ad6bf4da..3b694c50e 100644
--- a/packages/testing-runtime/src/Executor.mjs
+++ b/packages/testing-runtime/src/Executor.mjs
@@ -1,5 +1,5 @@
 import jwt from "jsonwebtoken";
-import { InlineFile, File } from "@teamkeel/functions-runtime";
+import { InlineFile, File, Duration } from "@teamkeel/functions-runtime";
 
 export class Executor {
   constructor(props) {
@@ -128,7 +128,9 @@ async function parseInputs(inputs) {
   if (inputs != null && typeof inputs === "object") {
     for (const keys of Object.keys(inputs)) {
       if (inputs[keys] !== null && typeof inputs[keys] === "object") {
-        if (isInlineFileOrFile(inputs[keys])) {
+        if (isDuration(inputs[keys])) {
+          inputs[keys] = inputs[keys].toISOString();
+        } else if (isInlineFileOrFile(inputs[keys])) {
           const contents = await inputs[keys].read();
           inputs[keys] = `data:${inputs[keys].contentType};name=${
             inputs[keys].filename
@@ -150,6 +152,10 @@ function isInlineFileOrFile(obj) {
   );
 }
 
+function isDuration(obj) {
+  return obj && typeof obj === "object" && obj.constructor.name === "Duration";
+}
+
 function parseOutputs(data) {
   if (!data) {
     return null;