Use this calendar tool when the calendar system, inclusivity rules, time zone, recurrence convention, and holiday inputs are explicit.

Date

Required string. Calendar date in YYYY-MM-DD format.

The cited overview of Calendar date supplies background for the terminology and domain context used by this tool.^[1]

Determine the ISO-8601 week number and ISO week year of a calendar date. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

ISO week number

Returned integer. Week number from 1 to 53 per ISO-8601 rules.

ISO year

Returned integer. ISO week year owning the week.

• Civil calendars, holidays, daylight-saving transitions, month-end clamping, and jurisdiction-specific business-day rules can produce different valid interpretations.
• Use finite inputs in the displayed units, preserve source measurements and assumptions, and independently verify consequential decisions.

Store unambiguous ISO dates and zone identifiers, and verify official calendars or a standards-compliant scheduling library for operational use.

• ISO Week Number

  Return the ISO-8601 week number and week year for a calendar date.

• ISO Week Year Calculator

  Return the ISO-8601 week-numbering year that owns the week of an ISO calendar date.

• Weeks in Year Calculator

  Return the number of ISO-8601 weeks in a year, given an ISO year or an ISO date.