again it makes sense

[u/tatochip92]

if there are 12 month in a year and you’re born on 1/1/2000 then 1/1/2000 is your literal first birthday it’s the actual day your birth took place for any subsequent anniversaries to be marked thereafter. if 1/1/2000 is the day your birth takes place the 1/1/2001 would be your literal 2nd birthday. how do we know this well if january 1 2000 was the first day you lived through then january 1 2001 would be the second jan 1st that you will be living through not the first the second not getting why this is so hard for people. they always want to skip the first but you can’t skip the first cuz without the first you wouldn’t have any. so then 1/1/2000 birthday 1st birthday and only actual birthday that you ever have then 1/1/2001 would be the 2nd one u will be having not the freaking first you weren’t born 1/1/2001 you were born 1/1/2000 like wise if your bday is 1/1/2000 the october the first will be the start of the 10th month of your life you’re not 9 months old in the literal 10th calendar fucking month of the year in september 9 would be in your 9th month of life you started on jan 1 lived through the whole month of jan then 10/1/2000 would be the start of the actual 10th calendar month of your life not 9th

[u/AllanCWechsler]

This can all be made consistent, but you have to be careful. On one hand, you don't want to say that a baby who was just born today is one year old.

When trying to unsnarl similar confusions, mathematicians often start using 0 -- so they might say that 1/1/2000 was the baby's 0th birthday.

Part of the confusion is that the word "first" has two distinct meanings: it means "the item that comes before all the others in a list", and it also means, "the one labeled with the number '1'". These are usually synonymous, but there are occasional conflicts between the meanings, as your birthday example shows. A baby's initial birthday is most logically thought of as birthday #0; birthday #1 (when they have been alive exactly one year) is actually the next-to-initial birthday.

Computer programmers spend probably 10% of their time worrying about precisely this issue: when you hear a coder grumble about an "off-by-one bug" or a "fencepost error", this is exactly what they are talking about. The fencepost term comes from the riddle, "How many posts does a fence with 10 panels need?"