Calendar Tools
Calendar tools describe the structure of the calendar itself: which weeks a date belongs to, how long a month or year is, when a period starts and ends, and how leap years fit in.
They are useful for planning, reporting periods and anything that has to line up with the Gregorian calendar and ISO 8601 week numbering.
Tools in this category
Week Number Find the ISO 8601 week number and week-year for any date. Leap Year Check whether a year is a leap year and find the previous and next leap years. Days in a Year Find out whether a year has 365 or 366 days. Days in a Month Look up how many days a month has in any year. Calendar Browse a clean calendar for any month and year, with week numbers. Start and End of Period Find the first and last day of the week, month, quarter or year containing a date.
Which tool do you need?
- What does a month look like, with week numbers? The Calendar shows any month with ISO week numbers and a choice of first weekday.
- Which ISO week is this date in? Week Number gives the ISO week and week-year, including year-end edge cases.
- Is a year a leap year? Leap Year applies the Gregorian rule and explains the answer.
- Does a year have 365 or 366 days? Days in a Year also lists each month's length.
- How many days does a month have? Days in a Month handles February correctly.
- When does the week, month, quarter or year containing a date begin and end? Start and End of Period lists all five ranges at once.
Good to know
The site uses the proleptic Gregorian calendar for every year, including years before its introduction in 1582. That is the convention of ISO 8601 and of JavaScript dates, but it differs from the Julian calendar that was in use in much of Europe before then.
Background reading
- The Gregorian calendar The calendar most of the world uses for civil dates: where it came from, how it differs from the Julian calendar, and how dates before 1582 are handled.
- Leap years The three-part Gregorian leap-year rule, why century years are different, a list of leap years, and what 29 February does to date arithmetic.
- Days in each month A table of month lengths, the day of the year each month starts on, quarter lengths, and how to remember which months have 30 or 31 days.
- ISO week numbers How ISO 8601 numbers weeks, why a year can have 53 weeks, why 1 January can belong to the previous year, and how ISO weeks differ from other week numbering.