[TypeScript]実行環境のタイムゾーンによらず日本時間で日付を操作する

const JST_OFFSET_MS = 9 * 60 * 60 * 1000;

export const getJSTFullYear = (date: Date): number => {
  const jstDate = new Date(date.getTime() + JST_OFFSET_MS);
  return jstDate.getUTCFullYear();
};

export const getJSTMonth = (date: Date): number => {
  const jstDate = new Date(date.getTime() + JST_OFFSET_MS);
  return jstDate.getUTCMonth(); // 0〜11
};

export const getJSTDate = (date: Date): number => {
  const jstDate = new Date(date.getTime() + JST_OFFSET_MS);
  return jstDate.getUTCDate();
};