Week Numbers and Countdowns: ISO Weeks, Business Days and Time Left
Money, health & time · October 17, 2026 · 1 min read
Planners, payroll and project schedules often talk about weeks by number, such as week 14, and deadlines by how many days remain. The rules behind both are short.
ISO week numbers
The ISO system, used across Europe and in most business calendars, starts each week on Monday. Week 1 is the week that contains the first Thursday of the year. That rule means early January can belong to the last week of the previous year, and late December to week 1 of the next. A year has 52 or 53 weeks, and years like 2020 and 2026 have 53.
The US system
The US system starts weeks on Sunday and puts January 1 in week 1, so its numbers can differ by one. If a number matters, say which system you are using. The Week Number Calculator shows both for any date, and also turns a week number back into the Monday and Sunday it covers.
Counting down to a date
The Countdown to a Date tool shows the days, hours, minutes and seconds left, as well as the weeks and the number of weekdays from Monday to Friday. Weekdays are useful for deadlines, but they don't know about public holidays, so subtract those yourself.
Where countdowns go wrong
- Time zones: a deadline at midnight is a different moment in each place, so state the time zone.
- Daylight saving: a day can be 23 or 25 hours long when the clocks change.
- Leap years: February has 29 days in 2024 and 2028, which changes counts that cross it.
- Counting rules: decide whether the first and last day count, because that changes the answer by one.
More date sums
To count the days between two dates, or to add months to a date, use the Date Calculator.