What Is an ISO Week Number?
If you’ve ever looked at a calendar and wondered why “Week 1” doesn’t always start on January 1st, you’ve encountered ISO week numbering. Here’s how it works and why it matters.
The ISO 8601 standard
The International Organization for Standardization (ISO) defines week numbering in ISO 8601. The rules are simple:
- Weeks start on Monday — not Sunday, as in some calendar traditions.
- Week 1 is the week containing the first Thursday of the year — or equivalently, the week containing January 4th.
- A year has either 52 or 53 weeks.
Why January 1st isn’t always in week 1
Because weeks start on Monday and week 1 must contain the first Thursday, some years have their first few days fall into the last week of the previous year.
For example, if January 1st is a Friday, Saturday, or Sunday, those days belong to week 52 or 53 of the prior year. This keeps weeks aligned and predictable.
Even and odd weeks
Many organizations use even/odd week numbering for scheduling:
- Even weeks (2, 4, 6, …) — one set of schedules or rotations
- Odd weeks (1, 3, 5, …) — the alternate set
This is common in European work schedules, school timetables, and waste collection calendars. In Swedish, this is known as “udda/jämn vecka” and in French as “semaine paire/impaire”.
How to calculate the week number
Most programming languages handle this natively:
// JavaScript
function getISOWeek(date) {
const d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
const dayNum = d.getUTCDay() || 7;
d.setUTCDate(d.getUTCDate() + 4 - dayNum);
const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
return Math.ceil(((d.getTime() - yearStart.getTime()) / 86400000 + 1) / 7);
}Week numbers on TimeDials
Every city page on TimeDials shows the current ISO week number along with whether it’s an even or odd week. You can also add week numbers to your embeddable clock widgets — just enable the “Show week number” option in the studio.