Birthday Calculator

Your birthday moves forward one weekday each year (two after 29 February). People born on 29 February have a real birthday only every four years – this calculator uses 28 February in other years.

How to use the Birthday Calculator

1. Date of birth
2. Count from

Results update as you type, and every field accepts the units shown in its menu.

How it works

Enter your date of birth and the date to count from (today by default).

Tips

• Check whether the end date should be included – event durations usually include it, elapsed time does not.
• Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
• Use 24-hour time or set AM/PM carefully when entering clock times.
• Time zones and daylight-saving changes are not applied unless the calculator says so.

Common mistakes to avoid

• Counting a 30-day month when a calendar month is meant (or the reverse).
• Forgetting leap years when counting days across February.
• Mixing decimal hours (7.5 h) with clock time (7:30).

Inputs explained

What each result means

• Current age = floor([Months]÷12)
• Days until your next birthday = [Next birthday] – [Count from]
• Next birthday falls on a = weekday([Next birthday])
• You will turn = round(([Next birthday] – [Date of birth])÷365.2425)
• You were born on a = weekday([Date of birth])
• Birthdays celebrated = floor([Months]÷12)