How to calculate your 10,000th day on Earth

As usual, whenever I try to take a nap I think of something to keep me awake. This time, it was my 10,000th day of life.

It’s pretty straightforward to calculate, though one needs to exercise some caution when converting it into a memorable format (for example, October 15, 2010).

To convert into years, multiply the following factors

(Note: Throughout this post, I multiply conversion factors instead of simply dividing 10000 by 365 because experience has taught me that division without understanding units can be very, very confusing in the end.)

Whoopty doo. I'm sure you'll be able to impress girls at a bar with this knowledge.

However, it’s more fun to know the specific date of your 10,000th day of life on Earth. This will require us to calculate more carefully. I’ve deliberately used a lengthy approach to highlight the steps, and make sure it’s understood why each correction factor is applied.


Ok, so from the above calculation we know it happens sometime between our 27th and 28th birthday. Now what?

First, let’s find out how many days are in 27 365-day years (the length of a common year).

Subtracting that from 10,000, we find out that there are

left to account for after the 27th common year.

How many 30-day months is that? Well, calculating it yields

Ok, so the number of days in 4 30-day months is

(I told you this would be overly detailed.)

Subtracting those 120 days from our remaining total yields

So, assuming 365-day years, and assuming 30-day months, the 10,000th day happens on your 27th year, 4th month, and 25th day on Earth.

BUT

We have to deal with two inconveniences:

1. Leap years
2. Months that do not have 30 days

1. Leap years

Because 27/4 = 6.75, it would be fair to assume that we have either 6 or 7 leap days (February 29ths) during those first 10,000 days.

But for completeness, I will point out that leap years do not automatically happen every 4 years. Every 100th year is skipped, but every 400th year is a leap year. For instance, 1700, 1800, and 1900 were not leap years, but 1600 and 2000 were leap years.

This won’t matter for anyone who isn’t from the future or over 110 years old, but, again, I’m being as detailed as I can be to avoid confusion.

2. Months that do not have 30 days
Unless you haven't been paying attention, some months have 31 days. February has either 28 (for a common year) or 29 (for a leap year).

Assuming you have all of your hand knuckles, you can keep track of which months have 31 days by counting from your left pinky knuckle. Including the spaces between knuckles (the "valleys"), this is a good way of keeping track which months have 31 days (the knuckles) and which have 30 or fewer days (the valleys). You use only the knuckles for your non-thumbs - when you get to your left index knuckle, you're supposed to continue with your right index knuckle (representing July and August, respectively, which each have 31 days).

from http://middleagefella.com/blog/?p=185

Example: calculating my 10,000th day.

I’m born on May 29, 1983.

I’ve lived through 7 leap days (February 29ths) – in 1984, 1988, 1992, 1996, 2000, 2004, and 2008.

Consequently, from my birth to my 27th birthday, I’ve lived

I’m born in May, so the months I have to consider to add four 30-day months are

June (30 days)
July (31 days)
August (31 days)
September (30 days)

Based on the months under consideration, I need to add an additional 2 days to the total for the first four months of the 28th year.

Finally, I figure out how many days I need to add to get to the end of my birth month. Since I'm born on the 29th, I need to add 2 days to get to the 31st.

Now let's figure out how many days are left.

With 14 days remaining, this means I’ll be 10,000 days old on the 14th day of the next month in sequence, which is October.

Therefore, I'll be 10,000 days old on October 14th.

This is great. Here I was thinking that the next important anniversary of my birth would be the 50th, when I start qualifying for senior discounts.

Update: If you're lazy/want to check your math, you can use the calculator at TKday.com.