diff --git a/homeassistant/helpers/device_registry.py b/homeassistant/helpers/device_registry.py
index 30001a64474..af0baa75a01 100644
--- a/homeassistant/helpers/device_registry.py
+++ b/homeassistant/helpers/device_registry.py
@@ -367,7 +367,7 @@ class DeviceEntry:
                     "config_entries": list(self.config_entries),
                     "configuration_url": self.configuration_url,
                     "connections": list(self.connections),
-                    "created_at": self.created_at.isoformat(),
+                    "created_at": self.created_at,
                     "disabled_by": self.disabled_by,
                     "entry_type": self.entry_type,
                     "hw_version": self.hw_version,
@@ -377,7 +377,7 @@ class DeviceEntry:
                     "manufacturer": self.manufacturer,
                     "model": self.model,
                     "model_id": self.model_id,
-                    "modified_at": self.modified_at.isoformat(),
+                    "modified_at": self.modified_at,
                     "name_by_user": self.name_by_user,
                     "name": self.name,
                     "primary_config_entry": self.primary_config_entry,
@@ -426,11 +426,11 @@ class DeletedDeviceEntry:
                 {
                     "config_entries": list(self.config_entries),
                     "connections": list(self.connections),
-                    "created_at": self.created_at.isoformat(),
+                    "created_at": self.created_at,
                     "identifiers": list(self.identifiers),
                     "id": self.id,
                     "orphaned_timestamp": self.orphaned_timestamp,
-                    "modified_at": self.modified_at.isoformat(),
+                    "modified_at": self.modified_at,
                 }
             )
         )
diff --git a/homeassistant/helpers/entity_registry.py b/homeassistant/helpers/entity_registry.py
index 6f4647030dd..df06a49e97f 100644
--- a/homeassistant/helpers/entity_registry.py
+++ b/homeassistant/helpers/entity_registry.py
@@ -338,7 +338,7 @@ class RegistryEntry:
                     "categories": self.categories,
                     "capabilities": self.capabilities,
                     "config_entry_id": self.config_entry_id,
-                    "created_at": self.created_at.isoformat(),
+                    "created_at": self.created_at,
                     "device_class": self.device_class,
                     "device_id": self.device_id,
                     "disabled_by": self.disabled_by,
@@ -349,7 +349,7 @@ class RegistryEntry:
                     "id": self.id,
                     "has_entity_name": self.has_entity_name,
                     "labels": list(self.labels),
-                    "modified_at": self.modified_at.isoformat(),
+                    "modified_at": self.modified_at,
                     "name": self.name,
                     "options": self.options,
                     "original_device_class": self.original_device_class,
@@ -420,10 +420,10 @@ class DeletedRegistryEntry:
             json_bytes(
                 {
                     "config_entry_id": self.config_entry_id,
-                    "created_at": self.created_at.isoformat(),
+                    "created_at": self.created_at,
                     "entity_id": self.entity_id,
                     "id": self.id,
-                    "modified_at": self.modified_at.isoformat(),
+                    "modified_at": self.modified_at,
                     "orphaned_timestamp": self.orphaned_timestamp,
                     "platform": self.platform,
                     "unique_id": self.unique_id,