The day your child is half your age
By Matthew Parrott, who makes Life Tally ·
There is exactly one day when a child is half a parent's age, to the day. Type two birthdays to find it, with the day your lives add up to a century.
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Why there's exactly one such day
Say you were 10,326 days old when your child was born. From then on you both count one day a day, so the gap between your counts stays 10,326 forever. On the day your child has lived 10,326 days, your count reads 20,652: exactly twice theirs. The day before, you were more than twice their age; the day after, less. It happens once, and never again.
In the example above, a parent born Thursday, June 2, 1988 and a child born Friday, September 9, 2016 reach it on Saturday, December 17, 2044, when the child is 28 years, 3 months and 8 days old. If it's your first child, that's also the day parenthood becomes half your life.
A hundred years, put together
Add two ages and the sum grows by two days every day. So it reaches a hundred years exactly fifty years after the midpoint of your two birthdays: in the example, Monday, July 22, 2052. For a parent and child it's often a day in the child's forties or fifties. For grandparents and grandchildren it can come much sooner.
Every shared day, named
Life Tally works these out for everyone you add, without being asked: the half-age days, the century day, the running count of days you've been alive together, and the day you'll have known someone longer than you ever didn't. Hold one person and carry them onto another to open Together and see them all on one braided line. For brothers and sisters, see your kids at the same age.
Checked against Life Tally 4.0 on October 27, 2026.
Questions
When will my child be half my age?
On the day your child has lived exactly as many days as there are between your two birthdays. Count the days from your birth date to your child's, then add that many days to your child's birth date. On that day your count is exactly twice theirs, and it never is again.
Is that the same day I'm twice my child's age?
Yes, the same day seen from the other side. Before it, a parent is more than twice the child's age; after it, less, and the ratio keeps shrinking toward one without reaching it.
Does it work for brothers and sisters, or a couple?
For any two people born on different days. For siblings it comes early: two children born three years apart reach it when the younger is three. For a couple born a few years apart, it came in early childhood.
What is the century day?
The day two lives add up to a hundred years. Each count rises by one a day, so together they rise by two, and the sum reaches a hundred years exactly fifty years after the calendar midpoint of the two birthdays. Life Tally names it for every pair you add, along with the round sums before and after it.
How many days have we been alive together?
Count from the younger person's birth date to today: every one of those days, you were both alive. Life Tally keeps that count for everyone you add, on the People tab and in Together.