Date.prototype.setUTCMonth()

const event = new Date("December 31, 1975 23:15:30 GMT-3:00");
console.log(event.toUTCString());
// 予想される結果: "Thu, 01 Jan 1976 02:15:30 GMT"
console.log(event.getUTCMonth());
// 予想される結果: 0
event.setUTCMonth(11);
console.log(event.toUTCString());
// 予想される結果: "Wed, 01 Dec 1976 02:15:30 GMT"

setUTCMonth(monthValue)
setUTCMonth(monthValue, dateValue)

const theBigDay = new Date();
theBigDay.setUTCMonth(11);