diff --git a/src/services/budgetService.ts b/src/services/budgetService.ts
index 2bcb48b..64f2c89 100644
--- a/src/services/budgetService.ts
+++ b/src/services/budgetService.ts
@@ -139,8 +139,14 @@ export async function updateBudget(id: number, data: Partial<BudgetFormInput>):
   if (data.categoryId !== undefined) payload.category_id = data.categoryId;
   if (data.accountId !== undefined) payload.account_id = data.accountId;
   if (data.isRolling !== undefined) payload.is_rolling = data.isRolling;
-  if (data.startDate !== undefined) payload.start_date = `${data.startDate}T00:00:00Z`;
-  if (data.endDate !== undefined) payload.end_date = data.endDate ? `${data.endDate}T23:59:59Z` : null;
+  if (data.startDate !== undefined) {
+    const dateStr = data.startDate.split('T')[0];
+    payload.start_date = `${dateStr}T00:00:00Z`;
+  }
+  if (data.endDate !== undefined) {
+    const dateStr = data.endDate ? data.endDate.split('T')[0] : null;
+    payload.end_date = dateStr ? `${dateStr}T23:59:59Z` : null;
+  }
 
   const response = await api.put<ApiResponse<ApiBudget>>(`/budgets/${id}`, payload);
   if (!response.data) {