Time is a concept that is very difficult to define, even though people have been trying for ages. For millennia, it has been a very fascinating topic of discussion in religion, science and philosophy. Throughout history, humans have invented many devices to help them track time. Whether that’s the time of day, day of the week or simply the year we are living in, having a sense of time is very important.

Measuring time

When it comes to measuring time, we use events in space-time as references. For example, one day is one rotation of the Earth around its axis, and one year is one rotation of the Earth around the Sun. So if someone is 25 years of age, that means they have survived for 25 full rotations of the Earth around the Sun. Of course, time would pass even if the earth stopped, however it is easier to understand time with these factors. One day lasts exactly 24 hours, or 1440 minutes, while one year lasts around 365 days, or 8760 hours.

The most basic unit of measuring time is a second. We can also use the SI system designated prefixes with seconds to describe larger or smaller time intervals. There is almost no reason to use the multiples, as we have more mainstream units for larger time intervals. Those are minutes, hours, days, weeks, months, years, decades, centuries and millennia.

When it comes ti using submultiples for seconds, they are very rarely used because they are so difficult to grasp. For example, 1 attosecond is 10^-18 seconds. That is exceptionally small, but how small is it actually? Let me give you a reference for this.

The speed of light is 2.998*10^8 m/s. An attosecond is so small, in 3 attoseconds, light can only travel from one side of an atom to the other. Knowing the diameter of an atom is 10^-5 m, we can easily prove this using math:

2.998*10^{8} \frac {m}{s} * 10^{-18}s = 2.998*10^{-10} s

Adulthood

Humans, during their lives, change a lot, both physically and psychologically. Recording time has made it easier for us to track these changes. For example, it is common knowledge that humans grow until they are about 25 years of age. At that point, they are in their physical prime.

Human aging

We find great importance in remembering important dates. Whether that’s your birthday, someone else’s birthday or a widely accepted holiday, you have to remember it. When it comes to birthdays specifically however, it goes without saying you have to know how old you are at all times. Many laws require you to be of certain age to do certain things.

In most of the world, after 18 years of age, a person will become an adult. There are a few countries, namely New Zealand, Japan, Taiwan and Thailand, where the age of adulthood is 20. When it comes to The USA, even though you are an adult at 18, you can drive at 16, but you can’t drink alcohol until you are 21.

How to calculate age from date of birth

Calculating your age using your date of birth without a calculator is not as difficult as it may sound. First you calculate the years, which is easy. You subtract the year you were born in from the current year, provided the day you were born has passed. Let me give you an example. If someone was born on June 25th 1986, on August 26th 2021, they would be:

2021 - 1986 = 35

So, they are 35 years old. Now all you have to do is calculate how many months and days. The process is the exact same for both. This person was born in June, and we are calculating for August. These are the 6th and 8th months. Subtracting them will get:

8 - 6 = 2

And now for the days:

26-25 = 1

As we can see, a person born on June 25th 1986 is going to be 35 years, 2 months and 1 day old on August 26th 2021.

How old was I on a certain date?

However, if we were calculating this person’s age on any date before June 25th, we would just have to go from the year before. Let’s see how old this person was on March 3rd 2021. Since, at that point, June 25th hasn’t yet passed, we calculate from 2020:

2020 - 1986 = 34

Now, for the months, all you have to do is calculate how many months are between June 2020 and February 2021. Keep in mind, we’re calculating till February, because the 26th day of March hasn’t yet occurred. The process would look like this:

(12-6)+2 = 6 + 2 = 8

Essentially, you take the month of birth, and subtract it from 12. In this case, it was June, which is the 6th month. To the result of that, you add the month you are in. In this case, it was March, which had not yet reached the 26th day, so we had to calculate from February, which is the 2nd month.

And for the days, it is once again the same process. You subtract the day of the month, in this case the 26th, from the number of days in the month. Since 2020 was a leap year, February had 29 days. To that, you add the day of the month you are in, and you are done:

(29-26) + 3 = 3 + 3 = 6

So, on March 3rd 2021, this person was 34 years, 8 months and 6 days old.

How old am I in days?

To calculate how old you are in days, you need to take everything you calculated before, and simply convert it. So, you would convert the years and months into days, and you would have your result.

A year has 365 days. You also need to track leap years. Leap years have 366 days. Let’s use the example from before, and convert 35 years, 2 months and 1 day into days:

35 \cdot 365 = 12775

Now, for the leap years, we need to check the calendar. 1988 was a leap year. Since they appear once every four years, this means that 1992, 1996, 2000, 2004, 2008, 2012, 2016 and 2020 were also all leap years. As we see, there have been 9 leap years since 1986. So, we need to add 9 to the result from before:

12775 + 9 = 12784

For the months, it is a bit more tedious. As we know, months have 31 or 30 days, with the exception of February, that has 28 days, and 29 on leap years. You need to keep track of this in order to be accurate. In our example, we have June and July, which have 30 and 31 days, respectively. Along with the 1 leftover day, our final result will be:

12784 + 31 + 30 + 1 = 12846

So, on August 26th 2021, this person was 12846 days old.

But why go through all that trouble, when you can simply use our calculator, which will give you even more information about your age? Our age calculator can calculate your exact age, down to minutes and even seconds! All you need to do is input your date of birth and the calculator will do the rest.

How old is the oldest person alive?

As of September 30th 2021, the oldest man alive is Saturnino de la Fuente García, according to Guiness world records. He was born on February 11th 1909 in Leon, Spain, making him over 112 years and 8 months old at the time of writing this. During his life, many wars took place but he managed to survive all of them and then some. He was not a solider, because of his short height. However, he was a shoemaker for the army, and contributed a lot to the cause.

The oldest person alive is Kane Tanaka, who was born on January 2nd 1903 in a village near Fukuoka, Japan. At the time of writing this, she is over 118 years and 9 months old. She is the 3rd oldest person to have ever lived, behind two more women, Sarah Knauss and Jeanne Calment, who is the oldest recorded person.

Jeanne Calment died in August of 1997, at the age of 122 years and 164 days. Her record stands untouched over 20 years later, with the second place being 119 years and 97 days.

All of them are supercentenarians. A supercentenarian is a person who has lived over 110 years of age or older. Statistically, 1 in every 1000 centenarians are supercentenarians. While still very rare, a much more common occurrence are centenarians. A centenarian is a person who has lived 100 years of age or older. The country with the largest number of centenarians is the US, with over 55000.

Average life expectancy

However, all of these people are very special because they managed to live so long, For the great majority, life will end a lot sooner.

Unsurprisingly, the highest average life expectancy is in Japan, averaging 84.2 years, while the lowest is in Lesotho, at 52.9 years. Generally, it the southern part of Africa, the life expectancy is below 65 years. For The USA, it is 78.5, and for Canada it is 82.8 years.

At the turn of the 20th century, the life expectancy was incredibly low by today’s standards, at only 46 years for men and 48 years for women. During the 20th century however, despite all the wars and epidemics, the life expectancy increased a lot. This is due to all the advancements in medicine, such as antibiotics and vaccines.

The first true antibiotic was discovered by Alexander Fleming in 1928, and was subsequently used in WW2, where the rates of soldiers dying from infections of wounds were greatly decreased. Vaccines for pertussis, diphtheria and tetanus, were all developed in the first half of the 20th century.