What is chronological age?

Calendar age since birth on a stated date. For an assessment, use the date the assessment took place. This calculator performs date arithmetic; it does not calculate biological or developmental age, test scores, norms, prematurity adjustments or eligibility.

How do I get the age on a test date?

A set date is already selected. Enter the birth date and As of date. For a birth date of 12 April 2018 and an assessment on 1 September 2026, the result is 8 years, 4 months and 20 days, or 100 completed calendar months. Both dates appear in the selectable and copied summary. Switching to Today measures age on the device’s current local date.

How are months and days counted?

Count completed calendar-month anniversaries from the birth date, then the days since the last anniversary. Short months clamp to their last day: 31 January 2025 to 1 March 2025 is 1 month and 1 day. The start date is age zero; the end date is not added as an extra day. This does not assume every month has 30 days and does not claim to match every test manual.

What happens with a 29 February birthday?

This tool uses 1 March in a common year and 29 February in a leap year. A birth date of 29 February 2004 therefore gives 20 years, 11 months and 30 days on 28 February 2025 and 21 years on 1 March. Apply a different convention separately if the assessment requires one.

What if the dates are reversed or incomplete?

A measured date before birth is refused with an explanation. Dates are not swapped. Both fields accept real Gregorian dates from 0001-01-01 through 9999-12-31; year zero and impossible dates are refused. With a birth date but an empty As of field, the answer is explicitly age today.

Are dates saved or sent anywhere?

The calculation runs in the browser. A deliberate valid birth-date change saves it locally; restoration is read-only. As of is not saved. Forget saved birth date removes the saved value without clearing the current calculation. Copy age includes both dates and the conventions; Selectable age summary provides the same text if copying is refused.

Chronological Age Calculator

Enter the date of birth and the date on which you want the age measured. This page opens with A set date selected. It gives Gregorian calendar age in years, months and days, plus whole-month, week and day totals. An empty As of field uses today. Check the required anniversary and rounding rules separately when a test manual or form specifies them.

Age calculatorPick a date of birth to see an exact age.

Date of birth

Age at

As of

Changing a valid birth date saves it in this browser. The As of date is not saved.

Set the date you want the age measured on: a test date, an assessment date, a deadline. While it is empty the answer is the age today.

Chronological age on 1 September 2026, by date of birth

Computed with the tool's own calculation for a fixed assessment date of Tuesday 1 September 2026, for children born on the first of each month over one year. It shows the shape of the answer rather than being a lookup table: put your own two dates into the tool above for the exact figure. Whole calendar months are counted first and the days left over after that, which is why one month of difference is not always the same number of days.

Date of birth	Age on 1 September 2026	In months	In days
September 1, 2020	6 years	72 months	2,191 days
October 1, 2020	5 years, 11 months	71 months	2,161 days
November 1, 2020	5 years, 10 months	70 months	2,130 days
December 1, 2020	5 years, 9 months	69 months	2,100 days
January 1, 2021	5 years, 8 months	68 months	2,069 days
February 1, 2021	5 years, 7 months	67 months	2,038 days
March 1, 2021	5 years, 6 months	66 months	2,010 days
April 1, 2021	5 years, 5 months	65 months	1,979 days
May 1, 2021	5 years, 4 months	64 months	1,949 days
June 1, 2021	5 years, 3 months	63 months	1,918 days
July 1, 2021	5 years, 2 months	62 months	1,888 days
August 1, 2021	5 years, 1 month	61 months	1,857 days

Common questions

What is chronological age?: Calendar age since birth on a stated date. For an assessment, use the date the assessment took place. This calculator performs date arithmetic; it does not calculate biological or developmental age, test scores, norms, prematurity adjustments or eligibility.
How do I get the age on a test date?: A set date is already selected. Enter the birth date and As of date. For a birth date of 12 April 2018 and an assessment on 1 September 2026, the result is 8 years, 4 months and 20 days, or 100 completed calendar months. Both dates appear in the selectable and copied summary. Switching to Today measures age on the device’s current local date.
How are months and days counted?: Count completed calendar-month anniversaries from the birth date, then the days since the last anniversary. Short months clamp to their last day: 31 January 2025 to 1 March 2025 is 1 month and 1 day. The start date is age zero; the end date is not added as an extra day. This does not assume every month has 30 days and does not claim to match every test manual.
What happens with a 29 February birthday?: This tool uses 1 March in a common year and 29 February in a leap year. A birth date of 29 February 2004 therefore gives 20 years, 11 months and 30 days on 28 February 2025 and 21 years on 1 March. Apply a different convention separately if the assessment requires one.
What if the dates are reversed or incomplete?: A measured date before birth is refused with an explanation. Dates are not swapped. Both fields accept real Gregorian dates from 0001-01-01 through 9999-12-31; year zero and impossible dates are refused. With a birth date but an empty As of field, the answer is explicitly age today.
Are dates saved or sent anywhere?: The calculation runs in the browser. A deliberate valid birth-date change saves it locally; restoration is read-only. As of is not saved. Forget saved birth date removes the saved value without clearing the current calculation. Copy age includes both dates and the conventions; Selectable age summary provides the same text if copying is refused.

Gregorian calendar age with whole-month anniversaries, March 1 for February 29 in common years, and short-month clamping. No prematurity adjustment, decimal-age conversion or test-specific norm calculation.