Original GreenCalc editorial illustration for the leap-year age guide. Calendar pages and a date picker connect the article to the Age Calculator and its February 29 setting.
Why two age calculators can disagree
A date difference in days is usually straightforward. Turning it into years, months and days introduces a choice: how should a whole month end when the following month has fewer days? January 31 has no matching day in February. A February 29 birthday has no matching date in most years.
Those missing dates explain some differences between tools. Before treating a result as an error, check the convention. The GreenCalc age calculator states its month-end method and lets you select a February 29 anniversary rule.
What happens to a February 29 birthday?
A Gregorian leap year has a February 29. Years divisible by four are usually leap years; century years must also be divisible by 400. That makes 2000 a leap year and 2100 a common year. A person’s actual date of birth does not change when the following year lacks February 29.
For calculation purposes, GreenCalc offers March 1 or February 28 as the anniversary in a non-leap year. The choice changes completed age at the boundary. Someone born on February 29, 2004 reaches the selected 2025 anniversary on February 28 under one option and March 1 under the other. In 2028, both options use February 29.
This is a calculation setting. It does not establish a legal birthday rule for every place or organization. If you are checking a deadline, use the rule in the relevant instructions and follow the steps in our cutoff-date age guide.
January 31 to February 28: is that one month?
In GreenCalc, January 31 to February 28, 2025 counts as one complete month. The tool keeps the starting day number when it exists, and otherwise uses the final day of the destination month. January 31 to March 30 therefore produces one month and 30 days, while January 31 to March 31 produces two months.
This approach does not turn a month into a fixed number of days. February 2025 contains 28 days, while March contains 31. Calendar months describe positions on the calendar; total days describe the length of the interval.
Age in days answers a different question
From February 28 to March 1, 2024, two calendar days pass because February 29 falls between them. The same dates in 2025 are one day apart. The total-day result includes that difference automatically.
Dividing total days by 365 does not reliably give completed age. Leap days accumulate and birthdays occur at particular points in the year. For completed years, count anniversaries. For “How many days old am I?”, use the total-day result directly.
Why the tool does not promise exact seconds lived
A date of birth does not contain a time of birth. Even if you know that time, elapsed hours may depend on the timezone and daylight saving changes between the two moments. Multiplying calendar days by 24 leaves those details out.
GreenCalc labels hours, minutes and seconds as calendar-day equivalents. They are useful conversions of the day count, but they should not be read as a live stopwatch from birth. Its calendar-day calculation uses UTC date arithmetic internally to avoid a daylight saving change turning a local day into a fractional day.
Choose the result that fits your question
For a birthday, start with completed years. For a milestone such as 10,000 days old, read total days. For a form, use the requested reference date and the organization’s convention. When comparing two tools, enter the same dates and check how each handles February 29 and shorter months.
Try the examples in the age calculator, or return to Time & Date calculators to explore the topic.
By GreenCalc · Updated 2026-10-19