ISO 8601 Week Number Code Snippets

JavaScript

Pure ECMAScript, no dependencies. Returns 1–53.

function isoWeek(date) {
  const d = new Date(Date.UTC(
    date.getFullYear(),
    date.getMonth(),
    date.getDate(),
  ));
  // Thursday in current week decides the year.
  d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() || 7));
  const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  return Math.ceil(((d - yearStart) / 86400000 + 1) / 7);
}

console.log(isoWeek(new Date())); // current ISO week number

TypeScript (Intl-aware)

Returns both the ISO week number and the ISO week-year.

// Intl.DateTimeFormat doesn't expose ISO weeks; use the formula above.
export function isoWeek(date: Date): number {
  const d = new Date(Date.UTC(
    date.getFullYear(),
    date.getMonth(),
    date.getDate(),
  ));
  d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() || 7));
  const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  return Math.ceil(((d.getTime() - yearStart.getTime()) / 86_400_000 + 1) / 7);
}

export function isoWeekYear(date: Date): number {
  const d = new Date(Date.UTC(
    date.getFullYear(),
    date.getMonth(),
    date.getDate(),
  ));
  d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() || 7));
  return d.getUTCFullYear();
}

Python

isocalendar() returns ISO 8601 (year, week, weekday) since Python 3.9.

from datetime import date

# Python 3.9+: date.isocalendar() returns a named tuple
year, week, weekday = date.today().isocalendar()
print(f"{year}-W{week:02d}-{weekday}")

PHP

DateTime::format with `o` (ISO year) and `W` (ISO week).

<?php
$today = new DateTime("now", new DateTimeZone("America/Toronto"));
echo $today->format("o-\WW"); // ISO week-year + ISO week, e.g. 2026-W18

Java

java.time.WeekFields.ISO is the standards-compliant accessor.

import java.time.LocalDate;
import java.time.temporal.WeekFields;

LocalDate today = LocalDate.now();
int week = today.get(WeekFields.ISO.weekOfWeekBasedYear());
int year = today.get(WeekFields.ISO.weekBasedYear());
System.out.printf("%d-W%02d%n", year, week);

C#

System.Globalization.ISOWeek (.NET Core 3.0+).

using System.Globalization;

var today = DateTime.UtcNow;
int week = ISOWeek.GetWeekOfYear(today);
int year = ISOWeek.GetYear(today);
Console.WriteLine($"{year}-W{week:D2}");

Go

time.Time.ISOWeek returns (year, week) in ISO 8601.

package main

import (
  "fmt"
  "time"
)

func main() {
  year, week := time.Now().UTC().ISOWeek()
  fmt.Printf("%d-W%02d\n", year, week)
}

Rust (chrono)

chrono::NaiveDate::iso_week returns the ISO week.

use chrono::{Datelike, Utc};

fn main() {
  let now = Utc::now().date_naive();
  let iso = now.iso_week();
  println!("{}-W{:02}", iso.year(), iso.week());
}

Ruby

Date#cweek (calendar week) and Date#cwyear (calendar week year).

require "date"
puts Date.today.cweek  # ISO 8601 week number
puts Date.today.cwyear # ISO 8601 week-year

PostgreSQL

Postgres EXTRACT WEEK is ISO 8601 by default.

SELECT
  EXTRACT(ISOYEAR  FROM CURRENT_DATE) AS iso_year,
  EXTRACT(WEEK    FROM CURRENT_DATE) AS iso_week;

MySQL / MariaDB

Mode 3 forces ISO 8601 (week 1 = first week with 4+ days).

SELECT
  YEARWEEK(CURRENT_DATE, 3) AS iso_year_week, -- mode 3 = ISO 8601
  WEEK(CURRENT_DATE, 3)     AS iso_week;

SQL Server (T-SQL)

DATEPART(ISO_WEEK, ...) was added in SQL Server 2008.

SELECT
  DATEPART(ISO_WEEK, GETUTCDATE()) AS iso_week,
  DATEPART(YEAR, DATEADD(DAY, 26 - DATEPART(ISO_WEEK, GETUTCDATE()), GETUTCDATE())) AS iso_year;

BigQuery / Standard SQL

Standard SQL ISOWEEK / ISOYEAR.

SELECT
  EXTRACT(ISOWEEK FROM CURRENT_DATE())     AS iso_week,
  EXTRACT(ISOYEAR FROM CURRENT_DATE())     AS iso_year;

Excel

Excel 2013+ has ISOWEEKNUM. Older versions need a manual formula.

=ISOWEEKNUM(TODAY())
=YEAR(TODAY()-WEEKDAY(TODAY(),3)+3)  ' ISO week-year
="W" & TEXT(ISOWEEKNUM(TODAY()),"00")

Google Sheets

ISOWEEKNUM is supported natively.

=ISOWEEKNUM(TODAY())
=YEAR(TODAY()-WEEKDAY(TODAY(),3)+3)

Frequently asked questions