diff --git a/.changeset/proud-wombats-occur.md b/.changeset/proud-wombats-occur.md
new file mode 100644
index 000000000..ff42c96f3
--- /dev/null
+++ b/.changeset/proud-wombats-occur.md
@@ -0,0 +1,6 @@
+---
+'@example/openapi-ts-fetch': patch
+'@hey-api/client-fetch': patch
+---
+
+fix: preserve key name in Date transformation
diff --git a/packages/client-fetch/src/utils.ts b/packages/client-fetch/src/utils.ts
index f46dae2d5..d5ae551d8 100644
--- a/packages/client-fetch/src/utils.ts
+++ b/packages/client-fetch/src/utils.ts
@@ -153,7 +153,7 @@ const serializeObjectParam = ({
   value: Record<string, unknown> | Date;
 }) => {
   if (value instanceof Date) {
-    return value.toISOString();
+    return `${name}=${value.toISOString()}`;
   }
 
   if (style !== 'deepObject' && !explode) {
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-fetch-bundle/core/utils.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-fetch-bundle/core/utils.ts.snap
index f46dae2d5..d5ae551d8 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-fetch-bundle/core/utils.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-fetch-bundle/core/utils.ts.snap
@@ -153,7 +153,7 @@ const serializeObjectParam = ({
   value: Record<string, unknown> | Date;
 }) => {
   if (value instanceof Date) {
-    return value.toISOString();
+    return `${name}=${value.toISOString()}`;
   }
 
   if (style !== 'deepObject' && !explode) {
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-fetch-bundle_transform/core/utils.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-fetch-bundle_transform/core/utils.ts.snap
index f46dae2d5..d5ae551d8 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-fetch-bundle_transform/core/utils.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-fetch-bundle_transform/core/utils.ts.snap
@@ -153,7 +153,7 @@ const serializeObjectParam = ({
   value: Record<string, unknown> | Date;
 }) => {
   if (value instanceof Date) {
-    return value.toISOString();
+    return `${name}=${value.toISOString()}`;
   }
 
   if (style !== 'deepObject' && !explode) {