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Omni's days-old calculator quickly answers a specific question and calculates how many days old you are in the blink of an eye. Keep reading to learn:

  • How to calculate how old you are in days using an approximate formula;
  • How to calculate your age in days exactly.

We will give you some practical examples, too. What are you waiting for?

How to calculate how many days old you are: an approximation

Even though we use calendars daily, counting dates is not a trivial task at all! The structure of a calendar contains numerous pitfalls:

  • Leap years: years divisible by 44 contain 366366 days, an not 365365. However, if the year is not divisible by 400400, then it's not leap.
  • Historical changes in calendars: in the past, our dates changed drastically: we still have to count these changes, but only if our problem covers those specific days!
  • Months duration: the length of a month differs, and we even created mnemonics for it!

🔎 Thirty days hath September,
April, June, and November,
All the rest have thirty-one,
Except February, twenty-eight days clear,
And twenty-nine in each leap year.

So we have:

  • 77, 3131 days long months;
  • 44, 3030 days long months; and
  • 11, 2828 days long month.

Thus the average month's length is:

lmonth=(7 ⁣× ⁣31) ⁣+ ⁣(4 ⁣× ⁣30) ⁣+ ⁣2812=3651230.4\begin{split} l_\mathrm{month} &= \frac{(7\!\times\!31) \!+\! (4\!\times\!30)\! +\! 28}{12}\\ & = \frac{365}{12}\approx 30.4 \end{split}

This is enough to calculate how old you are in days or, at least, to give you an approximation. Say that you're born on the 2121 August, 19981998. Today's date is the 1717 January, 20232023. To calculate how old you are on this day, we first need to subtract the two dates. To do so, follow these steps:

  • Calculate the difference between the days of the current date and the day of your birthday. If the latter is greater than the former, carry over 30.430.4 from the months of the current date and reduce the corresponding month's number by one.
  • Subtract the months' numbers. If you need to carry over from the year, add 1212 to the current month's number.
  • Subtract the years.

The result of these three subtractions is a set of three numbers, corresponding to your age in years, months, and days. To convert these values in days, use the following operations:

  • Leave the days untouched;
  • Multiply the number of months by 30.430.4.
  • Multiply the number of years by 365.25365.25 (the average length of a year, considering the leap years).

Sum these three results to find your age in days.

You learned how to calculate how many days old you are approximately; what about learning how to do this the exact way: you can also understand how computers and our days-old calculator are performing the same task!

Days old calculations: exact way to calculate how many days old you are

Due to the issues we detailed above, it's impossible to find your exact age in days using simple subtractions. To do so, we need to convert the starting and final date to Julian day.

The Julian day is a continuous quantity that measures the difference in days between a given date and a reference date. The reference date was set to January the 1st, 4713 BC4713\ \mathrm{BC}. The conversion between the calendar date and the Julian date is given by this rather complex formula:

JDN=1461×Y+4800+M14124+367×(M212×M1412)123×Y+4900+M14121004+D32075\scriptsize \begin{split} \mathrm{JDN} &= 1461 \times \frac{Y + 4800 + \frac{M − 14}{12}}{4} \\[1em] &+\frac{367 \times (M − 2 − 12 \times \frac{M − 14}{12})}{12} \\[1em] &− \frac{3 \times \frac{Y + 4900 + \frac{M - 14}{12}}{100}}{4} + D − 32075 \end{split}

where:

  • DD — The day's number, from 11 to 3131;
  • MM — The month's number, from 11 to 1212; and
  • YY — The year.

The result of this formula is a single number. For example, the 1515 October 19781978, corresponds to the Julian day 24437962443796. To calculate your age in days, simply convert both your birthday and the current date in Julian day number, and subtract them: the result will be exact!

Our days old calculator in action: examples of calcualting how many days old you are

Say that you were born on September the 1st 19971997. Let's calculate how old you are in days on the 2020 January 20232023!

  1. Subtract the number of days: 201=1920-1= 19.

  2. Subtract the number of months. Since January has a smaller number than September, we carry over 1212 from the years: (1+12)9=4(1\textcolor{blue}{+12})-9 = 4.

  3. Subtract the years: (20231)1997=25(2023\textcolor{red}{-1})-1997=25.

  4. Multiply the number of months by 30.430.4: 4×30.4=121.61224\times30.4=121.6\approx122.

  5. Multiply the difference in years by 365.25365.25:

    25×365=9131.25913225\times 365=9131.25\approx9132

  6. Sum days, months in days, and years in days: 19+122+9132=927319+122+9132=9273

The result is slightly different than the one we calculate with our days-old calculator due to the many approximations we used in our formulas!

FAQ

How do I calculate my age in days?

To calculate your age in days, follow these easy steps:

  1. Calculate the difference between the current date and your birthday: find the values of years, months, and days.

  2. Multiply the difference in months by 30.4 (the length of an average month).

  3. Multiply the difference in years by 365.25 (the average length of a year).

  4. Sum the two previous results and the difference in days to calculate how old you are in days.

How do I calculate how many days old was I on this date?

To calculate how many days old you were on this date, you must:

  1. Calculate the difference in years, months, and days between the two dates.
  2. Convert the quantities from months and years to days by multiplying., respectively, by 30.4 (average length of a month) and 365.25 (average length of a year).
  3. Sum the difference in days and the converted value, and eventually round up the result.

What you found is an approximation of your age in days on that specific date. To find a more accurate result, use an online calculator.

Why does my calculated age in days is not correct?

The hand calculations for your age in days based on the difference between the current date and your birthday suffer from a small set of approximations:

  1. The changing length of the year: leap years have 366 days. You can approximate the length of a year to 365.25.
  2. The varying length of the months: the result depends on the pair of months you chose.

The age you calculate can even be ten days off from the real one. Use an online calculator or more accurate methods to find your age in days!

How many days old am I if I was born on June 14, 1978?

If you were born on June 14, 1978, and today is January 18, 2023, you would be, approximately, 16288 days old. To find this result:

  1. Subtract your birthday from the current date:
    • For the days: 18 - 14 = 4;
    • For the months carry over 12: (1 + 12) - 6 = 7;
    • For the years we have: 2022 - 1978 = 44.
  2. Convert the last results in days:
    • 7 months are approximately equal to 7 × 30.4 ≈ 213 days;
    • 44 years corresponds to about 44 × 365.25 ≈ 16071 days.
  3. To find your age in days, sum these results: 4 + 213 + 16071 = 16288.
Davide Borchia
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