Short answer
Convert a date to a week number
Find the ISO week, weekday and week-year for any valid date.
Calculate a week number
Inputs are calendar dates without a time. ISO classification is therefore independent of the device time zone.
Short answer
Dates use the unambiguous YYYY-MM-DD form before being classified under ISO 8601.
How it works
The algorithm anchors the date to its week’s Thursday to determine the correct week-year.
Choose the date and time zone
A date such as 2027-01-01 is a calendar day without a time or zone. This calculator classifies that exact day. The site's current-week answer instead uses Europe/Berlin, which may differ from your location around midnight.
Read the result as a week-year and week-number pair: 1 January 2027 is 2026-W53. Check the Monday and Sunday too. 29 February 2028 is valid; 29 February 2027 is rejected rather than silently shifted into March.