diff --git a/server/routers/agent.py b/server/routers/agent.py
index 309024d..422f86b 100644
--- a/server/routers/agent.py
+++ b/server/routers/agent.py
@@ -93,7 +93,7 @@ async def get_agent_status(project_name: str):
     return AgentStatus(
         status=manager.status,
         pid=manager.pid,
-        started_at=manager.started_at,
+        started_at=manager.started_at.isoformat() if manager.started_at else None,
         yolo_mode=manager.yolo_mode,
         model=manager.model,
         parallel_mode=manager.parallel_mode,
diff --git a/server/routers/devserver.py b/server/routers/devserver.py
index 673bc3e..18f91ec 100644
--- a/server/routers/devserver.py
+++ b/server/routers/devserver.py
@@ -129,7 +129,7 @@ async def get_devserver_status(project_name: str) -> DevServerStatus:
         pid=manager.pid,
         url=manager.detected_url,
         command=manager._command,
-        started_at=manager.started_at,
+        started_at=manager.started_at.isoformat() if manager.started_at else None,
     )
 
 
diff --git a/server/routers/schedules.py b/server/routers/schedules.py
index 7c6c4ed..2a11ba3 100644
--- a/server/routers/schedules.py
+++ b/server/routers/schedules.py
@@ -256,8 +256,8 @@ async def get_next_scheduled_run(project_name: str):
 
         return NextRunResponse(
             has_schedules=True,
-            next_start=next_start if active_count == 0 else None,
-            next_end=latest_end,
+            next_start=next_start.isoformat() if (active_count == 0 and next_start) else None,
+            next_end=latest_end.isoformat() if latest_end else None,
             is_currently_running=active_count > 0,
             active_schedule_count=active_count,
         )