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JavaScript Date and Time

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

TL;DR

  1. 01Create dates using new Date with several input options.
  2. 02Read date parts with getFullYear(), change them with setFullYear().
  3. 03Format dates with toISOString() for machines and toLocaleDateString() for humans.
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Tips

  1. 01For complex date math like time zones, recurring events, or relative time, use a library like date-fns or Luxon.
  2. 02Store dates as ISO strings or timestamps in APIs and databases, converting to a Date object only for display.

Warnings

  1. 01JavaScript months are zero-indexed, so passing 0 means January and passing 11 means December, not month 12.
  2. 02Date string parsing varies by browser for non-ISO formats, so prefer ISO 8601 or new Date(year, month, day) instead.

Creating Dates

    new Date()

    The Date constructor accepts four different argument shapes for creating dates.

    const now = new Date();
    From milliseconds

    Pass a timestamp in milliseconds to create a date from a number.

    const fromMs = new Date(1697040000000);
    From a string

    Pass a date string to parse a specific date in standard format.

    const d = new Date("2025-10-11");
    From parts

    Pass year, month (zero-indexed), day, and time parts to build a custom date.

    const custom = new Date(2025, 9, 11, 15, 30);
    Checking validity

    Detects an invalid date by testing whether getTime() returns NaN.

    Number.isNaN(d.getTime()); // true if invalid

Reading Parts

    getFullYear()

    Reads the four-digit year directly from a Date object instance.

    now.getFullYear(); // 2025
    getMonth()

    Reads the month as a zero-indexed number, where zero means January.

    now.getMonth(); // 9 means October
    getDate() and getDay()

    getDate() returns the day of the month; getDay() returns the weekday.

    now.getDate();
now.getDay(); // 0 = Sunday
    Time components

    Use getHours(), getMinutes(), and getSeconds() to read the time components.

    now.getHours();
now.getMinutes();
now.getSeconds();
    getTime()

    Reads the date as a millisecond timestamp since the Unix epoch.

    now.getTime(); // ms since epoch

Changing Parts

    setFullYear()

    Every setter mutates the original Date object in place — there's no immutable version.

    const date = new Date();
date.setFullYear(2026);
    setMonth()

    Changes the month in place, again starting counting from zero.

    date.setMonth(0); // January
    setDate()

    Changes the day of the month, rolling over into next month.

    date.setDate(1);
    setHours()

    Use extra arguments to set minutes and seconds at the same time.

    date.setHours(12, 0, 0);
    setTime()

    Sets the entire date at once from a millisecond timestamp value.

    date.setTime(0); // Jan 1, 1970 UTC

Timestamps and UTC

    Date.now()

    Returns the current timestamp in milliseconds since the Unix epoch.

    const ts = Date.now();
    getUTC methods

    Reads times in universal coordinated time instead of the local timezone.

    now.getUTCFullYear();
now.getUTCHours();
    Easy comparisons

    Timestamps make it easy to compare or subtract two dates directly.

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

    Runs automatically inside JSON.stringify(), which produces an ISO date string.

    JSON.stringify({ d: now }); // ISO string
    new Date(ts)

    Convert any millisecond timestamp back into a usable Date object.

    const d = new Date(1700000000000);

Formatting and Math

    toISOString()

    Formats a date for machines, always returning UTC in a sortable format.

    now.toISOString();
// "2025-10-11T00:00:00.000Z"
    toLocaleDateString()

    Formats the date for the user's region for a human-readable display.

    now.toLocaleDateString();
    Adding days

    Read the current date and add to it to shift by any number of days.

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

    Subtract two dates to get the difference in milliseconds between them.

    const diffMs = date1 - date2; // milliseconds
    Converting units

    Convert milliseconds into days or hours using simple division for clarity.

    const hrs = ms / (1000 * 60 * 60);

In Practice

FAQ