Dates and time
ISO week number and date converter
Convert a Gregorian date to its ISO week date, or convert an ISO week-year, week and weekday back to a Gregorian date.
Runs in your browser. Inputs stay on this page and are not uploaded or added to the URL.
Your result
How to use this tool
Choose a direction and enter a Gregorian date or an ISO week date. The result includes both forms and the Monday/Sunday boundaries. ISO weeks start on Monday, and week 1 is the week containing 4 January, equivalently the first Thursday.
The ISO week-year can differ from the calendar year in late December and early January. Week 53 is accepted only when that week exists in the selected year. This is date arithmetic with no local time-zone conversion; invalid dates are rejected rather than normalized.
Input week-years and Gregorian dates are limited to years 0001–9999. Reverse conversion rejects a result outside that Gregorian range. For a valid date near the upper boundary, the displayed week may end in year 10000, using extended-year notation.
Worked example
2021-01-01 ↔ 2020-W53-5: Friday in ISO week 53 of week-year 2020. 2020-W01-1 converts to 2019-12-30.
References and method
Contifield is an independent utility site. Reference publishers do not endorse this tool.
Related tools
Excel serial date converter
Convert Excel serial numbers and dates using the explicit 1900 or 1904 system, including fractional days and Excel’s serial-60 exception.
Time duration calculator in UTC
Calculate elapsed time between two UTC dates or timestamps, with days, hours, minutes, seconds and milliseconds.
Unix timestamp converter
Convert Unix seconds or milliseconds to a UTC date, or convert an explicit UTC date/time into a Unix timestamp.