Weeks.so

Start and end of a week

Calculate the Monday and Sunday surrounding a date.

Find an ISO week

Select a date to display its ISO week number, ISO week-year and Monday-to-Sunday date range.

Enter or select values to calculate a result.

The calculation runs in your browser. Values entered here are not submitted to this site.

What it means

Calculate the Monday and Sunday surrounding a date.

How to calculate or use it

Subtract the date's ISO weekday minus one for Monday, then add six days for Sunday.

Important details

Some local calendars display Sunday first, so label the convention.

A reliable method

Calculate the Monday and Sunday surrounding a date. A dependable answer starts by defining the reference and convention before doing the arithmetic.

  1. Define the reference

    Use one complete Gregorian calendar date. Write it down when another person must be able to reproduce the answer.

  2. Check the inputs

    Enter the date and confirm whether the organization uses ISO Monday-based weeks or another local week convention.

  3. Apply the calculation

    Assign the date to the ISO week containing Thursday; week 1 is the week containing January 4.

  4. Label the result

    Show both week number and week-year because dates near New Year can belong to the adjacent ISO year.

Worked example

January 1 can fall in week 52 or 53 of the previous ISO year. December 31 can similarly belong to week 1 of the following ISO year.

How to interpret the answer

For start and end of a week, the useful answer includes its assumptions. Show both week number and week-year because dates near New Year can belong to the adjacent ISO year.

Accuracy checklist

  • Use a complete date whenever the result can cross midnight or a calendar boundary.
  • State the time zone or local-time basis instead of leaving it implied.
  • Label inclusive, exclusive, elapsed-time or calendar-based counting.
  • Recheck future results when civil-time or organizational rules may change.

Common mistakes

  • Using a familiar shortcut without checking whether it matches the guide's calendar convention.
  • Sunday-based calendars, retail calendars and locale-specific week starts do not always match ISO 8601.
  • Sharing only the final number without the reference date, zone, unit or rounding rule.

Frequently asked questions

What is the quickest practical way to use start and end of a week?

Start with one complete Gregorian calendar date. Use the interactive tool on this page, then check the result against the method and limitations below.

What can make a ISO weeks result inaccurate?

Sunday-based calendars, retail calendars and locale-specific week starts do not always match ISO 8601.

Does the calculator send my entries to the server?

No. The interactive calculation runs in the browser. For a result you plan to share, record the relevant date, time zone and counting convention alongside it.

Related ISO weeks guides