Calendar Calculators

Date Arithmetic, Weekday Finder, ISO Week Numbers and Leap Year Checker

Calendar calculators for everyday date arithmetic: add or subtract days from a date, find the weekday of any date, look up the ISO week number, check a leap year, count the days left in the year, and find the date of your next birthday or your retirement.

Every Calendars tool

Add Days To A Date

Add or subtract days, weeks, months or years from any date.

Day Of The Week Calculator

What day of the week any date falls on, past or future.

Days Left In The Year

Days left in the year, day of the year and percentage gone.

Leap Year Checker

Is it a leap year, which rule decides it, and the next one.

Week Number Calculator

The ISO week number for today or any date.

How the calendar calculators work

Each one counts real calendar days in the Gregorian calendar, so month lengths and 29 February are always handled for you. Adding months keeps the day of the month where it can and falls back to the last day of a shorter month, which is the same rule banks and spreadsheets use.

Dates without time zones

A date typed into these pages is read as a plain day, never as midnight in some other zone. That stops the classic bug where a date shifts by one for anyone west of Greenwich, and it means the same inputs give the same answer wherever you are.