Age Calculator
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The age calculator can find the amount of time between a birth/start date and a future date of interest on the calendar. The calculated age can be displayed in years, months, days, minutes, or seconds. The age display defaults to years. All dates assume the time of day is noon - for the birth date and the "age as of" date.
How We Calculate Age
To calculate the age, we use a technique to break down the birth date and the "age as of" date you enter into separate millisecond (ms) value. We then subtract the difference to get the time difference in ms. Next, based on the "display age in" selection, we divide those ms by the amount of ms in a minute, hour, day, etc. and display the age in an easy to read format using your chosen time interval.
How Many Days in Each Month
A well-known song can help you remember how many days each month has. It goes as follows: "Thirty days have September, April, June, and November. All of the rest, have 31, except February, which is all alone. It has 28 days and 29 on a leap year". Leap years are years cleanly divisibly by 4, with the exception of years divisible by 100 but not by 400, such as 2100. Normally, the same years as US Presidential elections and the Olympics. Leap years are needed because it takes the Earth approximately 365.25 days to revolve around the Sun.
Age Calculation Example
I want to know my exact age on the calendar day my younger brother was born. I was born on October 19, 1991 and my brother was born on February 18, 1994. The aforementioned method we use to calculate age gives millisecond (ms) values of 687,888,000,000 and 761,590,800,000, respectively as of Nov 5, 2025 at 6:20 pm EST. The ms values may seem arbitrary, but they are derived from Unix time - the number of milliseconds since January 1st, 1970. This is a computing mechanism that you don't really need to worry about for now. After calculating the difference in the two values and dividing by the necessary values to get a human readable age, we arrive at 2 years and 4 months. At this age, I don't remember when my brother was born, but it's nice to know exactly how much older I am than him and always will be.
