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WEEKNUM Function (LibreOffice Calc)

Date & Time Intermediate LibreOffice Calc LibreOffice 3.0

The WEEKNUM function returns the week number of a given date, using either system 1 (week starts on Sunday) or system 2 (week starts on Monday). It is ideal for locale-specific week numbering.

The **WEEKNUM** function returns the **week number** of a given date, using one of two numbering systems. Unlike ISOWEEKNUM, WEEKNUM is **locale-dependent** and allows you to choose whether weeks start on **Sunday** or **Monday**. It is ideal for regional reporting, business calendars, and systems that do not follow ISO‑8601.

What the WEEKNUM Function Does

  • Returns the week number (1–53)
  • Supports two numbering systems
  • Allows Sunday‑start or Monday‑start weeks
  • Works with real dates, serial numbers, and DATEVALUE

It is designed to be flexible, regional, and business‑friendly.

Syntax

WEEKNUM(date; [mode])

Arguments

  • date:
    A valid date or serial number.

  • mode (optional):
    Determines the week numbering system.

Numbering Systems

Mode Meaning Week Starts On
1 (default) System 1 Sunday
2 System 2 Monday

Important:
These are not ISO‑8601 rules. For ISO weeks, use ISOWEEKNUM.

Basic Examples

Default behavior (System 1, Sunday start)

=WEEKNUM("2024-01-01")

Monday‑start week numbering

=WEEKNUM("2024-01-01"; 2)

Using a cell reference

=WEEKNUM(A1)

Week number of today

=WEEKNUM(TODAY())

Advanced Examples

Week number from text using DATEVALUE

=WEEKNUM(DATEVALUE(A1))

Week number from imported CSV timestamp

=WEEKNUM(DATEVALUE(LEFT(A1;10)))

Week number from Excel serial date stored as text

=WEEKNUM(DATE(1899;12;30)+VALUE(A1))

Determine if two dates fall in the same week (System 2)

=WEEKNUM(A1;2)=WEEKNUM(B1;2)

Build a week label (e.g., “Week 05”)

="Week " & TEXT(WEEKNUM(A1;2);"00")

First day of the week (System 2)

=A1 - WEEKDAY(A1;2) + 1

Last day of the week (System 2)

=A1 - WEEKDAY(A1;2) + 7

Convert WEEKNUM to ISO week (approximation)

=ISOWEEKNUM(A1)

Determine the week’s year (System 2)

=YEAR(A1 - WEEKDAY(A1;2) + 1)

Differences Between WEEKNUM and ISOWEEKNUM

Feature WEEKNUM ISOWEEKNUM
Week starts Sunday (1) or Monday (2) Always Monday
Week 1 definition Locale/business rules Week with first Thursday
ISO‑compliant No Yes
Use case Regional calendars International standards

Edge Cases and Behavior Details

WEEKNUM returns an integer (1–53)

Accepts:

  • Real dates
  • Serial numbers
  • DATEVALUE outputs
  • ISO date strings

Invalid text → Err:502

Time components are ignored

WEEKNUM may produce different results depending on mode

WEEKNUM may differ from ISOWEEKNUM

WEEKNUM of an error → error propagates

Common Errors and Fixes

Err:502 — Invalid argument

Cause:

  • Text not recognized as a date
  • Non-numeric values

Fix:

  • Wrap with DATEVALUE
  • Clean text with TRIM or SUBSTITUTE

Wrong week due to mode confusion

Fix:

  • Use mode 2 for Monday‑start weeks
  • Use ISOWEEKNUM for ISO‑compliant numbering

Unexpected week at year boundaries

Cause:

  • WEEKNUM does not follow ISO rules

Fix:

  • Use ISOWEEKNUM if ISO behavior is required

Best Practices

  • Use WEEKNUM for regional or business calendars
  • Use ISOWEEKNUM for international reporting
  • Normalize text dates with DATEVALUE
  • Always specify mode explicitly for clarity
  • Use WEEKNUM with TEXT() to build week labels
WEEKNUM is your flexible, locale‑aware week‑numbering tool — perfect for business calendars, dashboards, and regional reporting.
  • Use ISOWEEKNUM for ISO‑8601 week numbering
  • Use WEEKDAY for day‑of‑week logic
  • Use DATE for constructing comparison dates
  • Use DATEDIF and DAYS for interval calculations

By mastering WEEKNUM and its companion functions, you can build powerful, flexible, and region‑aware date workflows in LibreOffice Calc.