在庫追加のカレンダー修正

frontend/src/pages/StockPage.tsx

@@ -30,6 +30,14 @@ import { STOCK_ERRORS } from '../constants/errorMessages';
 import DatePicker, { registerLocale } from 'react-datepicker';
 import { ja } from 'date-fns/locale/ja'; // date-fnsの日本語ロケールをインポート
 
+// 日付をyyyy-MM-dd形式で返す関数
+const formatDateLocal = (date: Date) => {
+  const year = date.getFullYear();
+  const month = (date.getMonth() + 1).toString().padStart(2, '0');
+  const day = date.getDate().toString().padStart(2, '0');
+  return `${year}-${month}-${day}`;
+};
+
 // 新規在庫の初期状態
 const EMPTY_STOCK: Omit<Stock, 'stockId' | 'stuffId'> & { stuffId: number | null } & { newAddition: boolean } = {
   stuffId: null,
@@ -494,7 +502,7 @@ const StockPage: React.FC = () => {
                   popperClassName="custom-datepicker-popper"
                   selected={newStock.buyDate ? new Date(newStock.buyDate) : null}
                   onChange={(date) =>
-                    setNewStock({ ...newStock, buyDate: date ? date.toISOString().split('T')[0] : '' })
+                    setNewStock({ ...newStock, buyDate: date ? formatDateLocal(date) : '' })
                   }
                   dateFormat="yyyy/MM/dd"
                   customInput={
@@ -521,7 +529,7 @@ const StockPage: React.FC = () => {
                   popperClassName="custom-datepicker-popper"
                   selected={newStock.expDate ? new Date(newStock.expDate) : null}
                   onChange={(date) =>
-                    setNewStock({ ...newStock, expDate: date ? date.toISOString().split('T')[0] : '' })
+                    setNewStock({ ...newStock, expDate: date ? formatDateLocal(date) : '' })
                   }
                   dateFormat="yyyy/MM/dd"
                   customInput={