Wrong Javascript Month After 2020

Question

Can anyone tell me why the month is March instead of Feb, if I enter a year after 2020? Tested on Windows 10, Chrome & Edge

  const date = new Date()
  date.setFullYear("2021")
  date.setMonth("1") // February
  date.setDate("23")
  console.log(date)

The result of console log...

Tue Mar 23 2021 22:38:13 GMT+0800 (Singapore Standard Time)

The following shows the correct month though...

var event = new Date('August 19, 2025 23:15:30');
event.setMonth(1);
console.log(event);

Result is

Wed Feb 19 2025 23:15:30 GMT+0800 (Singapore Standard Time)

Please Note: If I use year 2020 or below, the return values are correct. The answer needs to explain this anamoly...

Answer 1

Just to extend the comment ("2020 is a leap year, it has 29th of February. (I assume you are testing this code today, 29th of July)") with example code:

function test29(year){
  var date=new Date();
  date.setDate(29); // "emulate" today, 29th of something
  
  date.setFullYear(year);
  date.setMonth(1);
  date.setDate(23);
  console.log(date);
}
console.log("test29 is affected by leap years:");
test29(2019);
test29(2020);
test29(2021);
function test19(year){
  var date=new Date();
  date.setDate(19); // "emulate" a 19th of something
  
  date.setFullYear(year);
  date.setMonth(1);
  date.setDate(23);
  console.log(date);
}
console.log("test19 is not affected by leap years:");
test19(2019);
test19(2020);
test19(2021);

Plus what the pointed duplicate (JavaScript Date Bug February 2014) writes.