誕生日までの日数を計算する

var birth = "1981-5-14"
var [year, month, day] = birth.split("-");
var today = new Date();
var early_month = new Date(new Date().getFullYear(), month - 1, day);
var next_birth_year = early_month < today ? new Date().getFullYear() + 1 : new Date().getFullYear();

console.log(
  Math.ceil(
    (new Date(next_birth_year, month - 1, day) - today) / 86400000
  )
);