The code for calculating the second Sunday in March doesn't work this year (2026) because March starts with a Sunday.
// Second Sunday in March
const springForward = new Date(y, 2, 1 + (14 - new Date(y, 2, 1).getDay()));
Should probably be something like:
// Second Sunday in March
let firstDayInMarchDay = new Date(y, 2, 1).getDay();
let firstSundayInMarchDay = 1 + (7 - firstDayInMarchDay) % 7;
let secondSundayInMarchDay = firstSundayInMarchDay + 7;
const springForward = new Date(y, 2, secondSundayInMarchDay);
There's also a similar bug for November, which only is an issue when November 1st is a Sunday (also happens in 2026).