Enter a date of birth and the date you want the age on. You get the exact age, the age in months, weeks, days, hours, minutes and seconds, and the days until the next birthday.
Today's date is already filled in as the second date. Change it only when you want the age on a particular day, such as a start date at work. (If you leave it blank, today's date is used.)
Result
Enter a date of birth in the field on the left and press "Calculate". The result will appear here.
Table of Contents
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What you can do on this page
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What is this calculation used for?
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How to Use
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Formula
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Symbols and terms
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Good to know before you start
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How to calculate it in Excel
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How to calculate it in Google Sheets
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How to calculate it in Python
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How to write it in LaTeX and other math languages (copy and paste)
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How to have ChatGPT do the calculation
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DataChef Features
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Related Features
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NumberChef Calculators List
What you can do on this page
- Enter a date of birth and get the exact age in years, months and days on the spot. Change the second date to find the age on any day, such as the first day of a new job or the date of an exam
- The age is also shown in months, weeks, days (days since birth), hours, minutes and seconds, together with the number of days until the next birthday
- Use it to check the age to write on a form, a baby's age in weeks and months, or how many days have passed since an anniversary
- A plain-language explanation of how age is counted and copy-and-paste formulas for Excel, Google Sheets (the DATEDIF function) and Python are all on this page
This calculator uses the everyday rule that you turn one year older on your birthday. A few official rules count differently. For example, the Social Security Administration treats you as reaching an age on the day before your birthday, and rules for February 29 birthdays vary. See "Symbols and terms" at the bottom of the page for details.
What is this calculation used for?
Filling in your age on forms and applications
Many forms ask for your age "as of" a certain date, such as the date you submit an application or the first day of a program. Close to a birthday, the age on the day you fill in the form and the age on the deadline can be different, so it is safest to count the age on the date the form asks for.
The same calculation checks rules such as "must be at least 16 years old on the day of the test" or the "age as of" date that youth sports leagues use to place players in age groups.
Checking the kindergarten cutoff date
In most US states, a child starts kindergarten in the fall if they turn 5 by the state's cutoff date, which is often around September 1. Enter the child's date of birth and the cutoff date to see whether they will be 5 by then.
For example, a child born on September 3, 2021 is still 4 years old on September 1, 2026, so with a September 1 cutoff they would start kindergarten a year later. Cutoff dates vary by state and school district, so check your local rule.
Knowing when you reach a milestone age (voting, Medicare, Social Security)
Many rights and programs start at a set age: voting at 18, Medicare at 65, and full Social Security retirement benefits at 67 for people born in 1960 or later. Put in a future date to see how old you will be then, or how many days are left until your next birthday.
Be aware that some agencies count differently. The Social Security Administration treats you as reaching an age on the day before your birthday. Always confirm the exact rule with the official source before you plan.
Counting a baby's age in weeks and months (checkups and vaccines)
Well-child checkups and vaccine schedules are usually given as "at 2 months", "at 4 months" and so on, and for newborns people often count in weeks. A month is counted on the same day number in the next month (the corresponding date), so this calculator also gives the right answer for babies born at the end of a month, when "one month later" can be confusing.
The "Age in days" also counts the birth date as day 0, the usual way to count days since birth.
Counting the days since an anniversary (weddings, relationships, milestones)
"How many days since our wedding?" "When is our 1,000th day together?" Enter the anniversary date in place of the date of birth, and the same calculation works.
One thing to note: the "days" here are days elapsed, with the start date as day 0. If you want to celebrate "day 1,000" counting the start date as day 1, then 999 days elapsed is your "day 1,000". Decide which way you count before you plan the celebration.
Formula
Age in years
Standard notation
(the usual math form)
\(A\)
\(=\)
\(Y_2\)
\(-\)
\(Y_1\)
\(-\)
\(d\)
In words
(symbols replaced with words)
④
\(A\): age
\(=\)
①
\(Y_2\): year of the date
\(-\)
②
\(Y_1\): birth year
\(-\)
③
\(d\): adjustment (0 or 1)
The formula in words
①
Take the
\(Y_2\): year of the date
②
subtract the
\(Y_1\): birth year
③
then subtract the
\(d\): adjustment
(\(d = 1\) if this year's birthday has not come yet, \(d = 0\) on the birthday or after it)
④
and you get the
\(A\): age
Quick example
For someone born on January 15, 1990, the age on July 3, 2026 is (the birthday, January 15, has already passed that year, so the adjustment is 0)
\(A\): age
\(=\)
year of the date (2026)
\(-\)
birth year (1990)
\(-\)
adjustment (0)
\(2026 - 1990 - 0 = 36\)
Key idea
The adjustment \(d\) is the heart of this formula. The year subtraction \(2026 - 1990 = 36\) gives the age after the 2026 birthday, so until the birthday comes, you subtract 1 more. For the same person on January 1, 2026, the birthday (January 15) has not come yet, so the age is \(2026 - 1990 - 1 = 35\).
This is the everyday rule, where you turn one year older on the birthday itself. Some official rules are slightly different, such as the Social Security rule of reaching an age on the day before the birthday (see "Symbols and terms").
Age in months (total months)
Standard notation
(the usual math form)
\(M\)
\(=\)
\(12\)
\(\times\)
\(A\)
\(+\)
\(m\)
In words
(symbols replaced with words)
④
\(M\): total months
\(=\)
②
\(12\): months in a year
\(\times\)
①
\(A\): age in years
\(+\)
③
\(m\): extra months
The formula in words
①
Take the
\(A\): age in years
②
multiply it by
\(12\): months in a year
to turn years into months
③
add the
\(m\): extra months
(the months part of "X years Y months")
④
and you get the
\(M\): total months
Quick example
For someone who is 36 years, 5 months and 18 days old, the age in months is
\(M\): total months
\(=\)
months in a year (12)
\(\times\)
age (36 years)
\(+\)
extra months (5)
\(12 \times 36 + 5 = 437\)
Key idea
The answer in this example is "437 months and 18 days". The leftover days (18 here) cannot be turned into months, because months have different lengths (28 to 31 days). So they are kept as days.
A month is counted as complete when you reach the same day number in the next month (the corresponding date). For what happens when the next month has no such day, as with January 31, see "corresponding date" in the glossary.
Days in weeks, hours, minutes and seconds (unit conversion)
Standard notation
(the usual math form)
\(T_h\)
\(=\)
\(24\)
\(\times\)
\(N\)
\(T_m\)
\(=\)
\(1440\)
\(\times\)
\(N\)
\(T_s\)
\(=\)
\(86400\)
\(\times\)
\(N\)
\(N\)
\(=\)
\(7\)
\(\times\)
\(W\)
\(+\)
\(r\)
In words
(symbols replaced with words)
③
\(T_h\): length in hours
\(=\)
②
\(24\): hours in a day
\(\times\)
①
\(N\): days elapsed
\(T_m\): length in minutes
\(=\)
\(1440\): minutes in a day
\(\times\)
\(N\): days elapsed
\(T_s\): length in seconds
\(=\)
\(86400\): seconds in a day
\(\times\)
\(N\): days elapsed
\(N\): days elapsed
\(=\)
\(7\): days in a week
\(\times\)
④
\(W\): weeks
\(+\)
⑤
\(r\): extra days
The formula in words
①
Take the
\(N\): days elapsed
②
multiply it by
\(24\): hours in a day
③
and you get the
\(T_h\): length in hours
(multiply by 1440 for minutes, or by 86400 for seconds)
④
For weeks, divide instead. Divide the days elapsed by 7, and the whole-number answer is the
\(W\): weeks
⑤
and the remainder is the
\(r\): extra days
Quick example
For a baby who is 100 days old (\(100 \div 7 = 14\) remainder \(2\), so "14 weeks and 2 days")
\(T_h\): length in hours
\(=\)
hours in a day (24)
\(\times\)
days elapsed (100 days)
\(T_h = 24 \times 100 = 2400\)
\(T_m = 1440 \times 100 = 144000\)
\(T_s = 86400 \times 100 = 8640000\)
\(100 = 7 \times 14 + 2\)
Key idea
A day has 24 hours, an hour has 60 minutes, and a minute has 60 seconds. So to get minutes you multiply by \(24 \times 60 = 1440\), and to get seconds you multiply by \(24 \times 60 \times 60 = 86400\).
Weeks are the one case that uses division instead of multiplication. A week has 7 days, so divide the days elapsed by 7. The whole-number answer is the number of weeks and the remainder is the extra days (for example, 100 days is 14 weeks and 2 days).
The days elapsed \(N\) is the actual number of calendar days (days since birth, with the birth date counted as day 0). So February 29 in a leap year is counted as a day too.
Age comes down to one rule: subtract the years, and if that year's birthday has not come yet, subtract 1 more. For the age in months, weeks or hours, count the days since birth, and the rest is just multiplying or dividing by fixed numbers.
Symbols and terms
Symbols
| \(A\) | A | The age in years, from the first letter of "age". |
| \(Y_1\), \(Y_2\) | Y sub 1, Y sub 2 | The birth year (\(Y_1\)) and the year of the date you want the age on (\(Y_2\)). (Example - born in 1990, age in 2026: \(Y_1 = 1990\), \(Y_2 = 2026\)) |
| \((M_2, D_2) < (M_1, D_1)\) | comparing month-day pairs | Means "the month and day of the date (\(M_2, D_2\)) come before the month and day of the birthday (\(M_1, D_1\))". Compare the months first, and compare the days only when the months are the same - the same way words are ordered in a dictionary. |
| \(d\) | d | The adjustment in the age formula. It is 1 if that year's birthday has not come yet, and 0 on the birthday or after it. |
| \(M\), \(m\) | M (capital and lowercase) | The total months (\(M\)) and the extra months (\(m\)). For "36 years 5 months", \(m = 5\) and the total is \(M = 12 \times 36 + 5 = 437\) months. |
| \(N\) | N | The number of days elapsed since birth (days since birth). It counts the actual calendar days, with the birth date as day 0. |
| \(W\), \(r\) | W, r | The weeks (\(W\)) and the extra days (\(r\)). Divide the days elapsed by 7. The whole-number answer is \(W\) and the remainder is \(r\). |
| \(T_h,\ T_m,\ T_s\) | T sub h, T sub m, T sub s | The elapsed time written in hours, minutes and seconds. The small letter at the lower right (the subscript) is the first letter of the unit. |
Terms
| chronological age | Age counted from the date of birth, starting at 0 and going up by 1 on each birthday. It is the age used in everyday life and on official documents. Doctors and schools also use this name to tell it apart from other measures such as developmental age. |
| corresponding date | The same day number in a later month or year. For January 15, the corresponding date in the next month is February 15. The extra months in an age are counted by whether this date has been reached. When the next month has no such day, as with January 31, the last day of that month takes its place, and the month is complete when that last day ends. For example, a baby born on January 31, 2023 is "0 months 28 days" old on February 28, 2023 (not yet 1 month) and "1 month 1 day" old on March 1. The extra days are then counted again from the substitute day (February 28), so the age jumps from "0 months 28 days" straight to "1 month 1 day", and there is no day that reads "exactly 1 month 0 days". |
| leap year | A year with a February 29. It usually comes every 4 years, but years divisible by 100 are common years unless they are divisible by 400 (2100 is not a leap year, 2000 was). In this calculator, a person born on February 29 turns one year older on March 1 in common years. Laws and agencies do not all agree (some use February 28), so check the official rule when a legal age matters. |
| cutoff date | The date by which a child must reach a certain age, for example to start kindergarten. In most US states a child starts kindergarten in the fall if they turn 5 by the state's cutoff date, which is often around September 1 but varies by state. Children with birthdays just after the cutoff wait a year and become the oldest in their class. |
| days since birth | The number of days counted with the birth date as day 0. The "Age in days" in this calculator uses this count. It is 1 less than a count that calls the birth date "day 1", so be careful which one you need. |
| Social Security age rule | The US Social Security Administration treats a person as reaching an age on the day before their birthday. So for its rules, someone born on January 2 reaches the age on January 1. This calculator uses the everyday rule (a year older on the birthday), so it can differ by one day for such rules. |
Good to know before you start
Here is what helps you use the calculation on this page with real understanding, not just by pressing the button.
If you get stuck, going back over these topics is the quickest way forward.
| Subtracting larger numbers (Grade 4) |
|
| Time and the calendar (Grades 3–4) |
|
| Multi-digit multiplication (Grades 4–5) |
|
| Division with remainders (Grade 4) |
|
How to calculate it in Excel
Copy the whole table below and paste it into cell A1 in Excel. It works as is.
Table to find the exact age (years, months, days)
| Date of birth | 1990-01-15 |
| Age on this date | 2026-07-03 |
| Age (years) | =DATEDIF(B1,B2,"Y") |
| Extra months | =DATEDIF(B1,B2,"YM") |
| Extra days | =DATEDIF(B1,B2,"MD") |
Table for the age in months, weeks and days
| Date of birth | 1990-01-15 |
| Age on this date | 2026-07-03 |
| Total months | =DATEDIF(B1,B2,"M") |
| Total days (days since birth) | =B2-B1 |
| Weeks | =INT((B2-B1)/7) |
| Extra days (remainder after weeks) | =MOD(B2-B1,7) |
Table for the age in hours, minutes and seconds
| Date of birth | 1990-01-15 |
| Age on this date | 2026-07-03 |
| Age in hours | =(B2-B1)*24 |
| Age in minutes | =(B2-B1)*1440 |
| Age in seconds | =(B2-B1)*86400 |
Table for the age today (a trick that needs no second date)
| Date of birth | 1990-01-15 |
| Age today (years) | =DATEDIF(B1,TODAY(),"Y") |
Type dates in the form 1990-01-15 (year-month-day). Excel reads this form as a date whatever your regional settings are. The key is the DATEDIF function. Its third argument means: "Y" = age in completed years, "YM" = extra months, "MD" = extra days, "M" = total months. (DATEDIF does not appear in the list of suggested functions, but it has worked in every version of Excel.)
In the first table, B3 = 36, B4 = 5 and B5 = 18, so the age is "36 years, 5 months, 18 days". In the second table, the total is 13,318 days, which is 1,902 weeks and 4 days. If "=B2-B1" shows up as a date, change the cell format to "Number".
Note: DATEDIF with "MD" is known to be off by 1 or 2 days, or even to give odd values, around the ends of months (for example, for people born on the 31st). When the extra days matter, use this calculator's result as your guide.
With TODAY() as in the last table, the age updates to "age today" every time you open the file.
In the first table, B3 = 36, B4 = 5 and B5 = 18, so the age is "36 years, 5 months, 18 days". In the second table, the total is 13,318 days, which is 1,902 weeks and 4 days. If "=B2-B1" shows up as a date, change the cell format to "Number".
Note: DATEDIF with "MD" is known to be off by 1 or 2 days, or even to give odd values, around the ends of months (for example, for people born on the 31st). When the extra days matter, use this calculator's result as your guide.
With TODAY() as in the last table, the age updates to "age today" every time you open the file.
How to calculate it in Google Sheets
Copy the whole table below and paste it into cell A1 in Google Sheets. It works as is.
Table to find the exact age (years, months, days)
| Date of birth | 1990-01-15 |
| Age on this date | 2026-07-03 |
| Age (years) | =DATEDIF(B1,B2,"Y") |
| Extra months | =DATEDIF(B1,B2,"YM") |
| Extra days | =DATEDIF(B1,B2,"MD") |
Table for the age in months, weeks and days
| Date of birth | 1990-01-15 |
| Age on this date | 2026-07-03 |
| Total months | =DATEDIF(B1,B2,"M") |
| Total days (days since birth) | =B2-B1 |
| Weeks | =INT((B2-B1)/7) |
| Extra days (remainder after weeks) | =MOD(B2-B1,7) |
Table for the age in hours, minutes and seconds
| Date of birth | 1990-01-15 |
| Age on this date | 2026-07-03 |
| Age in hours | =(B2-B1)*24 |
| Age in minutes | =(B2-B1)*1440 |
| Age in seconds | =(B2-B1)*86400 |
Table for the age today (a trick that needs no second date)
| Date of birth | 1990-01-15 |
| Age today (years) | =DATEDIF(B1,TODAY(),"Y") |
In Google Sheets, DATEDIF, TODAY, INT and MOD work exactly as in Excel. Copy the whole table, paste it into cell A1, and replace the dates in B1 (and B2) with your own.
As in Excel, DATEDIF with "MD" can be off by 1 or 2 days around the ends of months.
As in Excel, DATEDIF with "MD" can be off by 1 or 2 days around the ends of months.
How to calculate it in Python
from datetime import date
birth = date(1990, 1, 15) # date of birth
ref = date(2026, 7, 3) # the date to find the age on (use date.today() for today)
# Age: subtract the years, then subtract 1 if the birthday has not come yet
age = ref.year - birth.year - ((ref.month, ref.day) < (birth.month, birth.day))
total_days = (ref - birth).days # days elapsed since birth
weeks, extra_days = divmod(total_days, 7) # weeks and extra days
print(f"Age: {age} years")
print(f"Days elapsed: {total_days} days ({weeks} weeks and {extra_days} days)")
print(f"Hours: {total_days * 24} / Minutes: {total_days * 1440} / Seconds: {total_days * 86400}")
Runs with the standard library only. The key is "(ref.month, ref.day) < (birth.month, birth.day)" in the age line. When Python compares (month, day) pairs, it compares the months first and the days only when the months are equal (the same order as words in a dictionary). So this part checks "the date comes before the birthday, so this year's birthday has not come yet". Python treats the result True/False as 1/0 in subtraction, so the adjustment in the formula fits in this one line.
How to write it in LaTeX and other math languages (copy and paste)
Age in years
A = Y₂ − Y₁ − d (d = 1 if the date is before that year's birthday, 0 on or after it)
A = Y_2 - Y_1 - d, \quad d = \begin{cases} 1 & ((M_2, D_2) < (M_1, D_1)) \\ 0 & (\text{otherwise}) \end{cases}
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<mi>A</mi><mo>=</mo>
<msub><mi>Y</mi><mn>2</mn></msub><mo>−</mo>
<msub><mi>Y</mi><mn>1</mn></msub><mo>−</mo>
<mi>d</mi><mo>,</mo><mspace width="0.7em"/>
<mi>d</mi><mo>=</mo>
<mrow>
<mo>{</mo>
<mtable columnalign="center left">
<mtr>
<mtd><mn>1</mn></mtd>
<mtd><mrow><mo>(</mo><msub><mi>M</mi><mn>2</mn></msub><mo>,</mo><msub><mi>D</mi><mn>2</mn></msub><mo>)</mo><mo><</mo><mo>(</mo><msub><mi>M</mi><mn>1</mn></msub><mo>,</mo><msub><mi>D</mi><mn>1</mn></msub><mo>)</mo></mrow></mtd>
</mtr>
<mtr>
<mtd><mn>0</mn></mtd>
<mtd><mtext>otherwise</mtext></mtd>
</mtr>
</mtable>
</mrow>
</mrow>
</math>
A = Y_2 - Y_1 - d, d = {(1, if (M_2, D_2) < (M_1, D_1)), (0, otherwise):}
age = y2 - y1 - Boole[m2 < m1 || (m2 == m1 && d2 < d1)]
age := y2 - y1 - piecewise(m2 < m1 or (m2 = m1 and d2 < d1), 1, 0);
age = y2 - y1 - (m2 < m1 | (m2 == m1 & d2 < d1));
A = Y_2 - Y_1 - d (d = 1 if the date is before that year's birthday, 0 on or after it)
Age in months (total months)
M = 12A + m
M = 12A + m
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<mi>M</mi><mo>=</mo>
<mn>12</mn><mi>A</mi>
<mo>+</mo><mi>m</mi>
</mrow>
</math>
M = 12A + m
totalMonths = 12*age + extraMonths
totalMonths := 12*age + extraMonths;
total_months = 12*age + extra_months;
M = 12A + m
Days in weeks, hours, minutes and seconds (unit conversion)
Tₕ = 24N, Tₘ = 1440N, Tₛ = 86400N, N = 7W + r (0 ≤ r ≤ 6)
T_h = 24N,\quad T_m = 1440N,\quad T_s = 86400N,\quad N = 7W + r\ \ (0 \le r \le 6)
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<msub><mi>T</mi><mi>h</mi></msub><mo>=</mo><mn>24</mn><mi>N</mi><mo>,</mo><mspace width="0.7em"/>
<msub><mi>T</mi><mi>m</mi></msub><mo>=</mo><mn>1440</mn><mi>N</mi><mo>,</mo><mspace width="0.7em"/>
<msub><mi>T</mi><mi>s</mi></msub><mo>=</mo><mn>86400</mn><mi>N</mi><mo>,</mo><mspace width="0.7em"/>
<mi>N</mi><mo>=</mo><mn>7</mn><mi>W</mi><mo>+</mo><mi>r</mi><mspace width="0.7em"/>
<mo>(</mo><mn>0</mn><mo>≤</mo><mi>r</mi><mo>≤</mo><mn>6</mn><mo>)</mo>
</mrow>
</math>
T_h = 24N, T_m = 1440N, T_s = 86400N, N = 7W + r (0 <= r <= 6)
hours = 24*days; minutes = 1440*days; seconds = 86400*days; {weeks, remDays} = QuotientRemainder[days, 7]
hours := 24*days; minutes := 1440*days; seconds := 86400*days; weeks := iquo(days, 7); remDays := irem(days, 7);
num_hours = 24*num_days; num_minutes = 1440*num_days; num_seconds = 86400*num_days; num_weeks = floor(num_days/7); rem_days = mod(num_days, 7);
T_h = 24N, T_m = 1440N, T_s = 86400N, N = 7W + r (0 ≤ r ≤ 6)
How to have ChatGPT do the calculation
You are an age calculation assistant. Do the following calculation by actually running Python code, and base your answer only on the numbers from the execution result (do not answer by mental math or guessing). The date of birth is January 15, 1990. For the date July 3, 2026, find each of the following: 1. The exact age in years, months and days (count a month as complete on the same day number in the next month; if that month has no such day, count the month as complete at the end of the month's last day) 2. The total number of days since birth (counting the birth date as day 0), and the same number written as "X weeks and Y days" 3. How many days are left until the next birthday (assume the age goes up by 1 on the birthday itself) Show the code you used and the numbers from the execution result.
How to Use
-
1Enter your numbersType the numbers you want to calculate with into the input fields
-
2CalculatePress the "Calculate" button
-
3Check the resultThe result appears on the spot. The same page also explains the idea behind the calculation and the formula
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