JavaScript Date Set Methods

Set methods change part of an existing Date: move it to a different day, change the hour, jump ahead a month. They are also the easiest way to do date math, because JavaScript handles month lengths and leap years for you.

// The Set Methods

const d = new Date(2026, 9, 2);   // Oct 2, 2026

d.setFullYear(2027);
d.setHours(9, 15);             // 9:15 AM

console.log(d.toString());

// Set Methods Change the Original

Unlike string methods, which return a new string, set methods modify the Date object itself. That matters when two variables point to the same Date:

const start = new Date(2026, 9, 2);
const end = start;               // NOT a copy: same object
end.setDate(10);

console.log(start.getDate());    // 10  (start changed too!)

Make a real copy first with new Date(original):

const start = new Date(2026, 9, 2);
const end = new Date(start);     // independent copy
end.setDate(10);

console.log(start.getDate(), end.getDate());   // 2 10

Several set methods accept extra, optional values. setFullYear can set the month and day at the same time:

const d = new Date("January 01, 2025");
d.setFullYear(2020, 11, 3);     // year, month, day
console.log(d.toDateString());

// Rollover: JavaScript Does the Calendar Math

Set a value past its normal range and the date rolls forward (or back) into the next unit automatically:

const d = new Date(2026, 0, 31);   // Jan 31
d.setDate(32);                    // "January 32nd"
console.log(d.toDateString());

// Adding Days

This rollover is what makes adding time simple. Read the current value, add to it, set it back:

function addDays(date, n) {
  const copy = new Date(date);
  copy.setDate(copy.getDate() + n);
  return copy;
}

const assigned = new Date(2026, 9, 2);
console.log(addDays(assigned, 30).toDateString());   // crosses into November
console.log(addDays(assigned, -2).toDateString());   // negative goes back

// The Day-Zero Trick: Days in a Month

Day 0 of a month rolls back to the last day of the previous month. That gives a one-line way to find how many days any month has, leap years included:

function daysInMonth(year, monthIndex) {
  return new Date(year, monthIndex + 1, 0).getDate();
}

console.log(daysInMonth(2026, 0));   // 31  (January)
console.log(daysInMonth(2026, 1));   // 28  (February)
console.log(daysInMonth(2028, 1));   // 29  (February, leap year)

// The Month-End Trap

Adding a month to January 31st asks for "February 31st," which does not exist, so it rolls over into March:

const d = new Date(2026, 0, 31);   // Jan 31
d.setMonth(d.getMonth() + 1);
console.log(d.toDateString());     // "Tue Mar 03 2026"  (not Feb 28!)

If you need "same day next month, or the last day if it does not exist," clamp the day using daysInMonth before setting it.

// Comparing Dates

const due = new Date(2026, 9, 2, 23, 59);
const turnedIn = new Date(2026, 9, 3, 8, 5);

if (turnedIn > due) {
  console.log("Late");
}

// Quick Check

const d = new Date(2026, 11, 30);   // Dec 30
d.setDate(d.getDate() + 5);
console.log(d.toDateString());

const a = new Date(2026, 0, 1);
const b = a;
b.setFullYear(2030);
console.log(a.getFullYear());

// Try It

▶ Open Playground

1. Click Open Playground. Move the copy one week ahead and confirm the original is unchanged
2. Use addDays to find the date 100 days from today
3. Print the number of days in every month of this year with a loop and daysInMonth
4. Set a date to midnight tonight with setHours(24, 0, 0, 0) and explain the result in your notes