Next Birthday Countdown

How it is calculated

The tool takes your date of birth and today's date, builds this year's birthday from the month and day, and counts the days until it. If this year's birthday has already passed, it moves it forward by one year. The age you are turning is the year of that coming birthday minus your birth year. Today counts as day zero, so on the birthday itself the countdown reads 0 days and the age is the one you are celebrating. The count uses real calendar days, so leap years are handled by the date functions. A 29 February birthday falls on 1 March in years that have no 29 February. The answer changes every day, because it is measured from the current date. Enter the birth date in year-month-day form.

Common mistakes

• Entering the date in day-month-year order, which can swap the day and month or be rejected. Use year-month-day.
• Entering today's date or the current year instead of the year of birth, which gives an age of 0 or 1.
• Expecting the count to include or exclude the birthday day. Today is counted as zero, so tomorrow's birthday shows 1 day.
• Reading a saved result later. The countdown is measured from the day you open it, so a number noted down last week is no longer correct.