Plain DatesClear date calculations, with the rules shown.

ISO week calculator

Find an ISO week number or date range

Convert a date to its ISO week-year and week number, or enter a week-year and week to find its Monday-through-Sunday range.

ISO week lookup

Why ISO week-year matters

The first and last few days of a calendar year can belong to an ISO week-year with a different number. Reporting only “week 1” without the week-year can therefore be ambiguous.

Boundary example

Monday, December 30, 2024 through Sunday, January 5, 2025 is ISO week 1 of 2025. Thursday anchors that week in 2025.

Week 53 validation

Some ISO week-years have 53 weeks and others have 52. If week 53 does not exist for the entered year, this calculator reports an error instead of silently returning dates from the following year.

Days Between DatesCompare two calendar dates with a consistent end-date rule.Add or Subtract From a DateMove a date by days, weeks, months, or years.Business Days CalculatorCount or add weekdays with optional custom holidays.Date CountdownCount full calendar days to a date and download an all-day calendar event.Age CalculatorCalculate age, birth weekday, and the next birthday.Day of the WeekFind the weekday, day of year, and ISO week for a date.Time CalculatorAdd or subtract hours and minutes with day-offset reporting.Unix Timestamp ConverterConvert seconds or milliseconds to UTC and local time.Printable CalendarGenerate a clean monthly calendar with either week-start convention.