JS Date Set

Reading a Date object's parts is only half the picture -- setFullYear(), setMonth(), setDate(), and their relatives let you modify an existing Date in place, changing just one part (like the year) while leaving the rest untouched, or even computing a date relative to another one.

In this page:

Setting the Year, Month, and Day
Setting Time Components
Computing Relative Dates
Setting a Date to the Start or End of a Period
Set Methods Return a Timestamp, Not a Date

Syntax

javascript

date.setFullYear(year);
date.setMonth(month);
date.setDate(day);
date.setHours(hours);

Setting the Year, Month, and Day

setFullYear(year), setMonth(month), and setDate(day) each modify one specific part of an existing Date object -- with setMonth() using the same zero-indexed convention (0 for January) as its getMonth() counterpart, a detail easy to trip over.

Note: Double-check setMonth() calls against the zero-indexed convention -- setMonth(0) sets January, not "month 0 which doesn't exist".

Warning: setDate() with a value beyond the month's actual length rolls over into the following month rather than raising an error, which can silently produce an unexpected date.

उदाहरण: Setting the Year, Month, and Day

javascript

const date = new Date(2024, 0, 1);
date.setFullYear(2025);
date.setMonth(5); // 0-indexed, 5 = June
date.setDate(15);
console.log(date.toDateString());

Setting Time Components

setHours(), setMinutes(), setSeconds(), and setMilliseconds() modify a Date's time-of-day portion, exactly the way the year/month/day setters modify its calendar portion -- useful for adjusting just the time while keeping the same calendar date.

Note: Use setHours() (and its relatives) to adjust just the time-of-day on a Date, without needing to reconstruct the entire object from scratch.

Warning: setHours() accepts optional additional arguments for minutes, seconds, and milliseconds all at once -- calling it with just one argument leaves the other time components unchanged, which may or may not be what you intend.

उदाहरण: Setting Time Components

javascript

// Declare the constant `date` as a new `Date` instance
// Declare the constant `date` as a new `Date` instance
const date = new Date();
// Call `date.setHours(10)`
// Call `date.setHours(10)`
date.setHours(10);
// Call `date.setMinutes(30)`
// Call `date.setMinutes(30)`
date.setMinutes(30);
// Call `date.setSeconds(0)`
// Call `date.setSeconds(0)`
date.setSeconds(0);
// Print `date.toTimeString()` to the console
// Print `date.toTimeString()` to the console
console.log(date.toTimeString());

Computing Relative Dates

A clever pattern combines a getter and setter of the same unit: date.setDate(date.getDate() + 7) computes 'one week from now' by reading the current day and adding 7, letting JavaScript's Date object handle any month or year rollover automatically.

Note: Let JavaScript's Date arithmetic handle month/year boundaries automatically by using this get-then-set pattern, rather than manually calculating "is this the last day of the month" yourself.

Warning: setDate() automatically rolls over into the next month (or even year) when the resulting day exceeds the current month's length -- this is the whole point of the pattern, not a bug.

उदाहरण: Computing Relative Dates

javascript

const date = new Date(2024, 0, 28);
date.setDate(date.getDate() + 7); // one week from now, handles rollover
console.log(date.toDateString());

Setting a Date to the Start or End of a Period

Combining setters lets you snap a date to a meaningful boundary -- setting the day to 1 to get the start of the month, or using setMonth(date.getMonth() + 1, 0) to get the last day of the current month, both common needs when building calendar or reporting features.

Note: Use setMonth(month + 1, 0) as a reliable pattern for finding a month's last day, since day 0 of the following month is always the last day of the current one.

Warning: A hardcoded assumption like "the month always has 30 days" breaks for February and any 31-day month -- always use Date-based calculation rather than a fixed guess.

उदाहरण: Setting a Date to the Start or End of a Period

javascript

const date = new Date(2024, 5, 15);
date.setDate(1); // start of the month
console.log(date.toDateString());
const lastDay = new Date(2024, 5, 15);
lastDay.setMonth(lastDay.getMonth() + 1, 0); // last day of the month
console.log(lastDay.toDateString());

Set Methods Return a Timestamp, Not a Date

Every set method returns the Date's new numeric timestamp (milliseconds since 1970) as its return value, not a new Date object -- the original Date instance itself is what got modified in place, and that same instance should be used going forward, not the setter's return value.

Note: Continue using the original Date variable after calling a setter, rather than trying to store or reuse the setter's numeric return value as if it were a Date.

Warning: Assigning the return value of a setter (const newDate = date.setFullYear(2030)) to a variable and then treating it like a Date object is a mistake -- it is just a plain number.

उदाहरण: Set Methods Return a Timestamp, Not a Date

javascript

const date = new Date(2024, 0, 1);
const result = date.setFullYear(2025); // returns a timestamp, not a Date
console.log(typeof result, result);
console.log(date.toDateString()); // use the original instance

Live Example

Chapter Quiz — Complete all 26 topics to unlock

0/26 topics done

Complete these topics first:

← Previous Lesson JS Dates Next Lesson → JS Math

JS Date Set

Setting the Year, Month, and Day

Setting Time Components

Computing Relative Dates

Setting a Date to the Start or End of a Period

Set Methods Return a Timestamp, Not a Date

Chapter Quiz — Complete all 26 topics to unlock

Login to run this code