calculateAge.ts

const calculateAge = (birthdayStr: string) => {
  if (!birthdayStr) return
  const birthday = new Date(birthdayStr)
  const today = new Date()
  let age = today.getFullYear() - birthday.getFullYear()
  const monthDiff = today.getMonth() - birthday.getMonth()

  // 誕生日が来ていない場合、年齢から1引く
  if (monthDiff < 0 || (monthDiff === 0 && today.getDate() < birthday.getDate())) {
    age--
  }
  return age
}