Date maths, finding midpoints?

March 20, 2021 1:38 PM

Or is it a midpoint? Probably not. Help me! I have run into this problem before and I can't get my head around it. I have a friend who is about half my age, I would like to find out when exactly we will be half/double each others ages.

Our birthdays are two days apart, some part of my brain thought it logical that the day between our birthdays should be it, but that can't be right even if it feels intuitive. For the sake of argument let's say they are 33 now and I am 66, and our birthdays are on the 12th and 14th of July.

Am I twice as old as them on the 13th of July?
There are midpoint calculators, but I can't think what I would plug in. I've previously tried to use this to work out exactly when I had known my wife half my life but couldn't manage it. Teach me!

posted by Iteki to Grab Bag (7 answers total)

Option 1: In this hypothetical, they were born on 12 July 1988 and you were born on Bastille Day 1955. This makes you 12052 days older. So you'll be twice as old as your friend on the day they're 12052 days old. This will happen on July 11, 2021.

Option 2: They were born on Bastille Day 1988 and you were born July 12 1955. This makes you 12056 days older, so you'll be twice as old when they're 12056 days old. This will happen 14 July 2021.

Links to put in real dates:
days in between your birthdates calculator
add days to their birthdate calculator
posted by GCU Sweet and Full of Grace at 1:57 PM on March 20, 2021 [1 favorite]

Just doing it brute force
the answer depends on whose birthday is first.
If you were born July 12 then on the year you turn 66
on July 12 you are exactly 66 and friend is 33 - 2 days.
On July 14, you are 66 +2days and friend is 33 +0 days
July 15, you are 66 +3, friend is 33 + 1
July 16, you are 66 + 4, friend is 33 +2


I think the way to generalize is to figure out how old exactly you were on the day friend was born and then double that. So, if friend was born July 12, you were 2 days shy of your 33rd birthday or 12043 days old (33*365, ignoring leap year days). You will be double that age at 24086 days old which, if we divide by 365 equals 65.989 years. Multiple 0.989 * 365 to convert to days, then you would be exactly double at 65 years and 361 days which would be July 10.
posted by metahawk at 1:58 PM on March 20, 2021

You can use this calculator to count how many days between two dates:

https://www.timeanddate.com/date/duration.html

So use this to find out how many days old you were, on your friends birthday.

Then use this calculator to find out when you are exactly twice his age (in days).

https://www.timeanddate.com/date/dateadd.html

By starting from their birthday and adding the number you found in step 1.
posted by muddgirl at 2:03 PM on March 20, 2021 [1 favorite]

Best answer: The only time you would be twice someone's age is when they reach the age you were when they were born.

If you convert it to days, it becomes much easier to work out.

July 12, 1988 is 12052 days from July 14, 1955.

So on July 11, 2021 (july 12, 1988 + 12052 days), you would be twice their age (July 14, 1955 + 24104 days).

You can plug in your own actual birthdates to be 100% accurate.
posted by madajb at 2:28 PM on March 20, 2021 [3 favorites]

Response by poster: Thanks, this all makes much more sense now :)
posted by Iteki at 2:31 PM on March 20, 2021

In general, it's an algebra problem. If A is Al's age at any given time, and B is Bo's age, and Al is n days older than Bo (works the same with years as with any other unit), you have

A = B + n

If you want to find when Al is twice as old as Bo, that's when A = 2 x B.

Solve those two together and you get B = n, which is to say that Bo is as old as the difference in their ages.

You can use this to solve any other similar question. When was Al ten times as old as Bo? A = 10 x B. Put that in the first equation and you get 9 x B = n, or B = n / 9, or when Bo was one ninth as old as the difference in ages. Born nine years apart? When Bo was one, Al was ten. Born 83.7 years apart? B = n/9 = 9.3, so when Bo is 9.3, Al will be 83.7+9.3 = 93, and ten times older.

Add more people, more qualifications, and it's straightforward to turn the numbers into variables and solve.

🌠Algebra!🌠
posted by alexei at 2:32 PM on March 20, 2021

You can do it in Excel: plug in today's date, the date of birth and subtract the second from the first. This will give you the age in days. For example: entering 2021/03/21 and subtracting 1980/01/01 tells you that someone born on New Year's Day 1980 is 15,055 days old.

Divide that by two and subtract that figure from 2021/03/21: hey presto, you'll see that someone born on August 10, 2000 is exactly half the age of the other person.

I occasionally play around with some dates to keep a sense of scale in time. I am a Gen-X sort and I note that in eight weeks (May 19, in fact), Kurt Cobain will have been dead longer than he was alive, a distinction that John Lennon passed last month.
posted by ricochet biscuit at 10:04 PM on March 20, 2021