MDN URL: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/setDate

The setDate documentation doesn't state that the month determination is performed in local time, rather than UTC.

Please add a line somewhere that states that the calculation is based on local time. The only way to know from the doc is to click through on the spec.

Specific section or headline? No.

Expected to see the calculation based on UTC (which it is not)

Tested with the following in UTC-8 timezone

const myDate = new Date('2021-01-01T00:00:00.000Z');
myDate.setDate(2);
console.log(myDate.toISOString()); // '2020-12-03T00:00:00.000Z'