ISOWEEKNUM

Returns the number of the ISO (International Organisation for Standardisation) week of the year where the provided date falls.

Sample usage

ISOWEEKNUM(DATE(1969, 7, 20))

ISOWEEKNUM("6/14/2002")

Syntax

ISOWEEKNUM(date)

• date – The date that determines the week number. Must refer to a cell containing a date, number, or function returning a date type.

Notes

• When entering the date, use the DATE function, as text values might return errors.
• Per the European system for numbering weeks (also known as ISO 8601):
  • Weeks begin on Monday and end on Sunday.
  • Week one of the year is the week containing the first Thursday of the year.
    • As a result, if the first Thursday is after 4 January, at least one of the following days, 1–3 January, will have an ISOWEEKNUM of 52 or 53.
    • Similarly, if the first Thursday is before 4 January, at least one of the following days, 29–31 December, will have an ISOWEEKNUM of 1. 
    • If the first Thursday is 4 January , 29–31 December of the previous year will have ISOWEEKNUM of 52 and 1–7 January of the current year will have ISOWEEKNUM of 1.

See also

• WEEKNUM: Returns a number representing the week of the year where the provided date falls.
• WEEKDAY: Returns a number representing the day of the week of the date provided.
• DAYS360: Returns the difference between two days based on the 360-day year used in some financial interest calculations.
• DATE: Converts a year, month and day into a date.
• DATEVALUE: Converts a provided date string in a known format into a date value.