Enter any date to find its ISO 8601 week number — plus the ISO week-year, the day of the week, and the day of the year. Weeks start Monday; week 1 holds the year's first Thursday.
ISO weeks run Monday to Sunday. Early-January dates may belong to the previous week-year. Dates use local time.
What does this tool do? It returns the ISO 8601 week number for any date, along with the ISO week-year, the day of the week, and the day of the year.
ISO 8601 numbers the weeks of the year from 1 to 52 or 53. Weeks start on Monday, and week 1 is the week that contains the year's first Thursday, which is the same as the week containing January 4.
Because a week belongs to the year that holds its Thursday, the first days of January can fall in the last week of the previous year. For that reason the tool also shows the ISO week-year, which may differ from the calendar year for dates near January 1.
The day of the year, or ordinal date, is the count of days from January 1, where January 1 is day 1 and December 31 is day 365, or day 366 in a leap year.