Why is my date off by one month?

JavaScript's Date constructor takes months as zero-indexed values, so January is 0 and December is 11. Always subtract 1 when passing a human-readable month number, e.g. new Date(2024, 0, 15) for January 15th.

How do I get the current Unix timestamp in JavaScript?

Use Date.now() to get milliseconds since the Unix epoch. Divide by 1000 and floor the result to get seconds: Math.floor(Date.now() / 1000) . You can also call new Date().getTime() for the same millisecond result.

How do I add days to a JavaScript Date?

Read the current day with getDate() , add your offset, then set it back with setDate() , e.g. date.setDate(date.getDate() + 7) . JavaScript automatically rolls over the month and year if the value exceeds the month's length. For anything more complex, like adding months or handling DST, use date-fns or Luxon.

How do I format a date as a localized string like '06/09/2026'?

Use toLocaleDateString() with a locale and options object: new Date().toLocaleDateString('en-US', { year: 'numeric', month: '2-digit', day: '2-digit' }) . For a sortable ISO format use toISOString() , which always returns UTC in the format YYYY-MM-DDTHH:mm:ss.sssZ.

Build / Code / JavaScript

JavaScript Date and Time

Create, format, and modify dates in JavaScript with timestamps, UTC, and common calculations.

TL;DR

Create dates using new Date with several input options.
Read date parts with getFullYear(), change them with setFullYear().
Format dates with toISOString() for machines and toLocaleDateString() for humans.

Jump to section

Creating Dates
Reading and Changing Parts
Timestamps and UTC
Formatting and Math
Tips
Warnings
In Practice
FAQ

Creating Dates

new Date()

Accepts four different argument shapes for creating dates.

const now = new Date();

From a string or parts

Parse a date string, or pass year/month/day/time parts.

const d = new Date("2025-10-11");
const custom = new Date(2025, 9, 11, 15, 30);

From milliseconds

Pass a millisecond timestamp to build a date from a number.

const fromMs = new Date(1697040000000);

Reading and Changing Parts

getFullYear() / setFullYear()

Reads the four-digit year, or sets it to change the date.

now.getFullYear(); // 2025
date.setFullYear(2026);

getMonth() / setMonth()

Reads or sets the month as a zero-indexed number (0 = Jan).

now.getMonth(); // 9 means October
date.setMonth(0); // January

getDate() / getDay() / setDate()

Day of month or weekday; setDate() changes it in place.

now.getDate(); now.getDay(); // 0 = Sunday
date.setDate(1);

Timestamps and UTC

Date.now()

Returns the current timestamp in milliseconds since the epoch.

const ts = Date.now();

getUTC methods

Reads times in universal coordinated time, not local timezone.

now.getUTCFullYear(); now.getUTCHours();

Time components

Read hours, minutes, seconds; setHours() sets all three.

now.getHours();
date.setHours(12, 0, 0);

Easy comparisons

Timestamps make it easy to compare or subtract two dates.

date1.getTime() < date2.getTime();

Formatting and Math

toISOString() / toLocaleDateString()

Formats a date for machines (UTC) or a readable local string.

now.toISOString();
now.toLocaleDateString();

Adding days

Read the current date and add to it to shift by any amount.

const today = new Date();
today.setDate(today.getDate() + 5);

Subtracting dates

Subtract two dates to get the difference in milliseconds.

const diffMs = date1 - date2; // milliseconds

Tips

For complex date math like time zones, recurring events, or relative time, use a library like date-fns or Luxon.
Store dates as ISO strings or timestamps in APIs and databases, converting to a Date object only for display.

Warnings

JavaScript months are zero-indexed, so passing 0 means January and passing 11 means December, not month 12.
Date string parsing varies by browser for non-ISO formats, so prefer ISO 8601 or new Date(year, month, day) instead.

In Practice

Creates a target date, computes the whole-day difference from today, and formats both dates for display.

new Date(year, month, day) builds the deadline — remember month is zero-indexed, so October is 9.
Subtracting two Date objects returns the difference in milliseconds, not days.
Dividing by the number of milliseconds in a day and rounding up gives a whole day count.
toLocaleDateString() formats each date for display while the raw Date objects handle the math.

const today = new Date();
const deadline = new Date(2026, 9, 31); // October 31, 2026 (month is zero-indexed)

const msPerDay = 1000 * 60 * 60 * 24;
const daysLeft = Math.ceil((deadline - today) / msPerDay);

console.log(`Today: ${today.toLocaleDateString()}`);
console.log(`Deadline: ${deadline.toLocaleDateString()}`);
console.log(`${daysLeft} day(s) remaining`);