This number calculator converts calendar dates to dates and converts dates back to the calendar.
Match a date to its ISO week year
Enter a calendar date to find its number, and weekday. An starts on Monday and ends on Sunday. The can differ from the near January 1, so keep it with the number when labeling a report. Switch the conversion to enter an , number and weekday when you need a calendar date. This tool uses the ISO scheme throughout; it does not infer a regional or fiscal numbering convention.
A calendar date around the start of January can belong to an before or after its .
start on Monday. Sunday-first, fiscal and employer-specific numbering can produce different labels.
Week-year boundary worked example
January 1, 2021 returns 53, 2020 and weekday 5, Friday. Its complete ISO notation is 2020-W53-5. The date is in 2021, while its belongs to the prior .
In the other direction, December 30, 2019 is 1 of 2020. The ordinary October 5, 2026 example returns 41 of 2026, Monday.
Why ISO week years cross January and December
1 contains the first Thursday of its . Every is assigned to the containing its Thursday. January 4 is therefore always in 1, which provides the Monday anchor used by the calculation. Early January can finish the prior , and late December can begin the next one.
An contains 52 or 53 complete . The tool checks the selected before accepting 53. A calendar date alone does not tell you which numbering system a business report expects. Confirm its convention before transferring an ISO label. The tool uses entered dates and ISO numbering throughout.
Convert an ISO week date back to a calendar date
Enter 2020, 53 and Friday. The inverse preset returns January 1, 2021. 2021, 1 and Monday returns January 4, 2021. 2021, 52 and Sunday returns January 2, 2022, across the boundary.
Inputs and converted calendar dates must stay in 0001 through 9999. The upper date 9999-12-31 is 52 of 9999. Its complete 's Sunday is January 2, 10000 (beyond supported calendar input range). The tool labels that derived endpoint explicitly and rejects an inverse selection that would itself fall outside the supported calendar input range.
Use the Date Calculator to move dates or count calendar intervals. Use the Business Days Calculator when eligible work-calendar dates determine the result. Elapsed clock intervals belong to the Time Duration Calculator. identity is a label for a date, rather than a duration between two dates.