What is a Chronological Age
Unlike biological age, which refers to how old you appear to be, chronological age is how long you have been alive, expressed in the number of years, months, and days.
For example, if you were born on October 5, 1996, your chronological age as of June 20, 2019 would be 22 years, 8 months, and 15 days.
How to Calculate Chronological Age by Hand
For times when you don't have access to a calculator like the one on this page, the following example illustrates how to figure a person's age in years, months, and days using simple subtraction (chronological age formula used by the calculator).
For our example, suppose the person/student being tested was born on November 30, 1996, and the date the test is being administered is August 7, 2013.
To arrive at the chronological age, we first set up a subtraction problem, where we will subtract the birth date from the test date. Here is how our initial subtraction problem is laid out:
| Date | Years | Months | Days | |
|---|---|---|---|---|
| Test | 2013 | 8 | 7 | |
| − | Birth | 1996 | 11 | 30 |
Step #1: As with any other subtraction problem, we start subtracting in the right-most column -- which in this case is the "Days" column:
| Date | Years | Months | Days | |
|---|---|---|---|---|
| Test | 2013 | 7 | 37 | |
| − | Birth | 1996 | 11 | 30 |
| = | Age | 7 |
Since 7 days in the Test row is less than 30 days in the Birth row, we need to borrow 30 days (1 month) from the Test/Months cell, which leaves us with 7 months in the Test/Months cell and 37 days in the Test/Days cell. Subtracting 30 days from 37 days gives us a result of 7 days in the Age/Days cell.
Step #2: Next we move to the Months column:
| Date | Years | Months | Days | |
|---|---|---|---|---|
| Test | 2012 | 19 | 37 | |
| − | Birth | 1996 | 11 | 30 |
| = | Age | 8 | 7 |
Since 7 months in the Test row is less than 11 months in the Birth row, we need to borrow 12 months (1 year) from the Test/Years cell, which leaves us with 2012 years in the Test/Years cell and 19 months in the Test/Months cell. Subtracting 11 months from 19 months gives us a result of 8 months in the Age/Months cell.
Step #3: Lastly, we perform the subtraction in the Years column:
| Date | Years | Months | Days | |
|---|---|---|---|---|
| Test | 2012 | 19 | 37 | |
| − | Birth | 1996 | 11 | 30 |
| = | Age | 16 | 8 | 7 |
Subtracting 1996 years from 2012 years gives us a result of 16 years in the Age/Years cell. Therefore, subtracting the birth date from the test date gives us a chronological age of 16 years, 8 months, and 7 days.
Taking the age calculation one step further, you can calculate the chronological age in months by rounding to the nearest month (16 years, 8 months, and 7 days rounded to 16 years, 8 months), convert the years to months (16 years x 12 = 192 months) and total the months (192 + 8 = 200 months).
Note that I intentionally used two dates that would require borrowing from one column to supplement another so you could see how the borrowing works when subtracting dates. In cases where the top number (test months or days) is greater than the bottom number (birth months or days), you would simply subtract the bottom number from the top number.