Knowing “What Week Are We In” might seem simple, but it’s a question with surprising depth, especially when you consider its importance in various fields from scheduling to data analysis. This article will delve into understanding week numbers, focusing on the widely accepted ISO 8601 standard, and provide practical methods to determine the current week using different programming languages and tools.
Understanding Week Numbers and the ISO 8601 Standard
The concept of week numbers helps organize the year into consistent, seven-day periods. The ISO 8601 standard is the globally recognized system for this, providing a clear and unambiguous method for referencing weeks. According to ISO 8601, a week starts on Monday and ends on Sunday. The first week of the year is defined as the week containing the year’s first Thursday, which is also equivalent to the week containing January 4th. This system ensures that week numbers are consistent across different years and avoids confusion that can arise with other week numbering conventions.
For example, Week 08 in 2025, as we will illustrate, spans from Monday, February 17, 2025, to Sunday, February 23, 2025. This standardization is crucial for international communication, business planning, and software development. The ISO week representation for this period is 2025-W08.
It’s important to note that while ISO 8601 is prevalent, other week numbering systems exist. Some systems, particularly in the US, consider Sunday as the first day of the week. However, for clarity and global compatibility, ISO 8601 is the recommended standard, and the one we will focus on throughout this guide. A year can have either 52 or 53 weeks based on the ISO standard, and 2025 is a 52-week year.
Programmatic Ways to Determine “What Week Are We In”
For developers and those needing to automate week number calculations, numerous programming routines are available. Here are code snippets in various languages, demonstrating how to programmatically find the current week number, often adhering to the ISO 8601 standard:
Spreadsheets: Microsoft Excel, LibreOffice Calc, and Google Sheets
Spreadsheet software provides built-in functions to calculate week numbers.
Excel/LibreOffice Calc:
=ISOWEEKNUM(TODAY())or, for older versions:
=WEEKNUM(TODAY(),21)The ISOWEEKNUM function directly returns the ISO week number. In older versions, using WEEKNUM(TODAY(),21) specifies that the week starts on Monday and uses the ISO 8601 definition. For Excel 2007, WEEKNUM(TODAY(),2) is a suitable alternative, also starting weeks on Monday. Avoid using WEEKNUM(TODAY()) in Excel if you require ISO week numbers, as it defaults to weeks starting on Sunday.
Google Sheets:
=WEEKNUM(TODAY(),21)Google Sheets uses the same WEEKNUM function with the type parameter, 21, to ensure ISO 8601 compatibility, mirroring Excel and LibreOffice.
Programming Languages
Various programming languages offer functions to retrieve the week number. Here are examples in popular languages:
PHP:
$weekNumber = date("W");The PHP date("W") function directly returns the ISO-8601 week number. Ensure you use the uppercase “W” for the ISO week.
Python:
import datetime
week_number = datetime.date.today().isocalendar()[1]Python’s datetime module, using isocalendar(), provides a tuple containing the ISO year, week number, and weekday. Index [1] retrieves the week number.
PERL:
my $weekNumber = POSIX::strftime("%V", gmtime time);Perl’s POSIX::strftime function with the format specifier “%V” returns the ISO 8601 week number.
Java:
import java.util.Calendar;
import java.util.Calendar;
Calendar now = Calendar.getInstance();
int weekNumber = now.get(Calendar.WEEK_OF_YEAR);Java’s Calendar class, specifically Calendar.WEEK_OF_YEAR, provides the week number. For ISO 8601 consistency, ensure the Calendar’s first day of the week is set to Monday and the minimum days in the first week are set to 4, if necessary for specific ISO adherence.
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 snippet extends the Date object to include a getWeek() function, implementing the ISO 8601 week calculation.
C#:
// Refer to Stack Overflow for ISO-8601 compliant methods:
// [https://stackoverflow.com/questions/11154673/get-the-correct-week-number-of-a-given-date](https://stackoverflow.com/questions/11154673/get-the-correct-week-number-of-a-given-date)C# offers various approaches, and the Stack Overflow link provides detailed, ISO-8601 compliant solutions.
MySQL:
SELECT WEEKOFYEAR(NOW());MySQL’s WEEKOFYEAR() function calculates the week number. For ISO 8601, you can also use WEEK(NOW(),3).
PostgreSQL:
SELECT EXTRACT(WEEK FROM current_date);PostgreSQL uses EXTRACT(WEEK FROM ...) to get the week number.
MS SQL Server:
SELECT DATEPART(wk, GETDATE());MS SQL Server’s DATEPART(wk, GETDATE()) function retrieves the week number.
Oracle:
SELECT to_char(sysdate, 'IW') FROM DUAL;Oracle’s to_char(sysdate, 'IW') with the ‘IW’ format mask returns the ISO week number.
iSeries SQL (IBM i):
SELECT WEEK(NOW()) from sysibm.sysdummy1;iSeries SQL uses the WEEK() function.
Swift (iOS/macOS):
let gregorian = Calendar(identifier: .gregorian)
gregorian.firstWeekday = 2 // Monday
gregorian.minimumDaysInFirstWeek = 4
let components = gregorian.dateComponents([.weekOfYear, .yearForWeekOfYear], from: Date())
let week = components.weekOfYear
let year = components.yearForWeekOfYearSwift utilizes Calendar components to accurately calculate the ISO week number, setting the firstWeekday to Monday and minimumDaysInFirstWeek to 4 for ISO 8601 compliance.
R:
library(lubridate)
week(Sys.Date())The lubridate package in R simplifies date and time manipulations, with week() function providing the week number.
Ruby:
week_number = Time.now.strftime("%V")Ruby’s strftime("%V") format specifier returns the ISO 8601 week number.
Go:
package main
import (
"fmt"
"time"
)
func main() {
year, week := time.Now().ISOWeek()
fmt.Println("Week:", week, "Year:", year)
}Go’s time package provides the ISOWeek() method to get both the ISO week number and year.
Linux/Unix Shell (bash):
date +%VThe date +%V command in Linux/Unix shells directly outputs the ISO 8601 week number.
Lua:
Current_week = os.date("%V")Lua’s os.date("%V") function, similar to Ruby and PHP, uses “%V” for the ISO week number.
Windows PowerShell:
Get-Date -UFormat %VPowerShell’s Get-Date -UFormat %V command provides the ISO week number.
X++ (Microsoft Dynamics AX):
int weeknum;
weeknum = weekOfYear(today());X++ in Dynamics AX uses the weekOfYear(today()) function.
C/AL (Microsoft Dynamics NAV/Business Central):
MESSAGE(FORMAT(CALCDATE('CW', TODAY), 0, '<week>'));C/AL in Dynamics NAV/Business Central uses CALCDATE('CW', TODAY) to calculate the week.
Conclusion
Understanding “what week are we in” is more than just a calendar curiosity. It’s a practical need for scheduling, data organization, and cross-platform compatibility. By adopting the ISO 8601 standard and utilizing the programming examples provided, you can accurately and consistently determine the week number in various applications and contexts. Whether you are managing project timelines, analyzing sales data by week, or simply curious about the current week, these tools and explanations offer a comprehensive guide to week numbering.
