Use this calendar calculation when dates follow the stated calendar and you need a reproducible boundary, interval, ordinal, week, or classification result.

Date

Calendar date.

Return the ISO-8601 week number and week year for a calendar date. The tool evaluates the supplied inputs together and returns the named outputs below; it does not infer omitted operating conditions or change the units shown.^[1]

Week

ISO week number.

Week year (years)

Year that owns the ISO week.

• Calendar dates are not elapsed-time instants by themselves. Time zones, daylight-saving transitions, locale-specific week rules, inclusive endpoints, holidays, historical calendar reforms, and proleptic-calendar assumptions can change an apparently simple result.
• Use finite inputs in the displayed units and preserve more precision than the final presentation requires. Independently verify safety-critical, financial, compliance, or production decisions.

Store instants with an explicit time zone when time of day matters, and document whether interval endpoints are inclusive. Supply a jurisdiction-specific holiday calendar for business-day decisions.

• ISO Week Year Calculator

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

• ISO Week Number Calculator

  Determine the ISO-8601 week number and ISO week year of a calendar date.

• Weeks in Year Calculator

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