Ever found yourself asking, “What Week Is It?” It might seem like a simple question, but understanding week numbers is surprisingly useful in today’s fast-paced world. Whether you’re managing projects, scheduling events, or just trying to stay organized, knowing the week number can be a handy tool. This guide will break down everything you need to know about week numbers, focusing on the internationally recognized ISO 8601 standard and how to easily find out the current week.
Understanding ISO 8601 Week Numbers
The most widely accepted system for numbering weeks is defined by ISO 8601, an international standard. This system provides a consistent and unambiguous way to refer to weeks throughout the year. Here’s what makes ISO 8601 week numbering unique:
- Weeks Start on Monday: In the ISO 8601 system, each week begins on a Monday and ends on a Sunday. This is different from some systems, like the US system, where weeks start on Sunday.
- First Week of the Year: The first week of the year is defined as the week that contains the year’s first Thursday. Another way to think of it is the week containing January 4th. This “first 4-day week” rule ensures that week 1 is always the first week with a majority of its days in the new year.
- Week Number Range: Week numbers in a year range from W01 to either W52 or W53, depending on the year. A year with 53 weeks occurs when January 1st falls on a Thursday (or Wednesday in common year, Thursday in leap year) and in years immediately following those. For example, 2025 has 52 weeks.
Using the ISO 8601 standard eliminates confusion when communicating dates across different regions and industries. It’s particularly important in business, logistics, and programming, where consistent date references are crucial.
For example, according to ISO 8601, Week 08 of 2025 begins on Monday, February 17, 2025, and ends on Sunday, February 23, 2025. The ISO representation for this week is 2025-W08.
Why Use Week Numbers?
Week numbers offer a practical way to:
- Schedule and Plan: Businesses use week numbers for production schedules, financial reporting, and project timelines. Instead of saying “the meeting in the third week of March,” you can simply say “Week 12,” removing any ambiguity.
- Data Analysis: In data analysis, week numbers help in aggregating data weekly, making it easier to spot trends and patterns over consistent seven-day periods.
- Programming and Development: As you’ll see below, many programming languages and software tools use week numbers for date and time calculations, especially when adhering to international standards.
Finding the Week Number Programmatically
For developers and tech-savvy individuals, calculating the week number programmatically is often necessary. Here are code snippets in various popular programming languages and tools to get the ISO 8601 week number:
Spreadsheet Software
Microsoft Excel / LibreOffice Calc
=ISOWEEKNUM(TODAY())or (for older versions of Excel):
=WEEKNUM(TODAY(),21)The return type ’21’ in WEEKNUM specifically designates the ISO-8601 week starting on Monday. For Excel 2007, WEEKNUM(TODAY(),2) also works for weeks starting on Monday. Be aware that WEEKNUM(TODAY()) without the second argument defaults to weeks starting on Sunday.
Google Sheets
=WEEKNUM(TODAY(),21)Similar to Excel and LibreOffice, using type ’21’ ensures ISO-8601 compatibility.
Scripting Languages
PHP
$weekNumber = date("W");Use uppercase ‘W’ in PHP’s date() function to retrieve the ISO week number.
Python
import datetime
week_number = datetime.date.today().isocalendar()[1]Python’s datetime module and isocalendar() method provide a straightforward way to get the ISO week number.
Perl
my $weekNumber = POSIX::strftime("%V", gmtime time);The POSIX::strftime function with format specifier “%V” in Perl returns the ISO week number.
JavaScript
Date.prototype.getWeek = function () {
var target = new Date(this.valueOf());
var dayNr = (this.getDay() + 6) % 7;
target.setDate(target.getDate() - dayNr + 3);
var firstThursday = target.valueOf();
target.setMonth(0, 1);
if (target.getDay() != 4) {
target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
}
return 1 + Math.ceil((firstThursday - target) / 604800000);
}
var d = new Date();
alert(d.getWeek());This JavaScript code snippet extends the Date object to include a getWeek() function for ISO week number calculation.
Ruby
week_number = Time.now.strftime("%V")Ruby’s strftime with format “%V” provides the ISO week number. Other formats like “%U” (week starting Sunday) and “%W” (week starting Monday, but different first week rule) are also available.
Go
package main
import (
"fmt"
"time"
)
func main() {
year, week := time.Now().ISOWeek()
fmt.Println("Year:", year, "Week:", week)
}Go’s time package includes the ISOWeek() method to directly obtain the ISO year and week number.
Lua
Current_week = os.date("%V")Lua’s os.date() function with format “%V” retrieves the ISO week number.
Database Systems
MySQL
SELECT WEEKOFYEAR(NOW());To get the week number for a specific date, replace NOW() with the date, e.g., SELECT WEEKOFYEAR('2025-02-20');. You can also use WEEK(NOW(),3) with mode 3 for ISO week numbers.
PostgreSQL
SELECT EXTRACT(WEEK FROM current_date());PostgreSQL’s EXTRACT(WEEK FROM ...) function extracts the week number.
MS SQL Server
SELECT DATEPART(wk, GETDATE());DATEPART(wk, ...) in MS SQL Server returns the week number.
Oracle
SELECT to_char(sysdate, 'IW') FROM DUAL;Oracle’s to_char(..., 'IW') function with the ‘IW’ format specifier gives the ISO week number. ‘WW’ is another format, but it uses a different week numbering system not based on ISO 8601.
iSeries SQL (DB2)
SELECT WEEK(NOW()) FROM sysibm.sysdummy1;Other Languages/Environments
Code snippets are also provided in the original article for C#, Java, Perl, C/AL (Microsoft Dynamics NAV), X++ (Microsoft Dynamics AX), iPhone/Mac (Objective-C), iPhone/iOS/Swift, R, Linux/Unix shell (bash), Windows PowerShell. Please refer to the original article for these code examples as they are already well-presented and comprehensive.
Conclusion
Understanding “what week is it” and the ISO 8601 week numbering system can significantly improve organization and communication, especially in international contexts and technical fields. Whether you need to schedule a meeting, analyze data, or write code that handles dates correctly, week numbers are a valuable tool. Use this guide and the code examples provided to easily determine the week number in any situation.
