History of Zero-based Months?

August 24th, 2022
history, tech
JavaScript is a language of many silly things, and one of them is:

> new Date()
Wed Aug 24 2022 ...
> new Date().getFullYear()
2022
> new Date().getMonth()
7
> new Date().getDate()
24

It represents 2022-08-24 as (2022, 7, 24). One-based indexing for the year and day, but zero-based indexing for the month.

In this case, however, the problem was copied from Java:

getMonth: The value returned is between 0 and 11, with the value 0 representing January.

getDate: The value returned is between 1 and 31 representing the day of the month.

I'd love to blame Java, but they seem to have copied the problem from C:

localtime(3):
tm_mday: The day of the month, in the range 1 to 31.
tm_mon:  The number of months since January, in the range 0 to 11.

Looking at the Unix History repo, the first mention of "month (0-11)" is in 1973's Research Unix V4:

The value is a pointer
to an array whose components are
.s3
.lp +5 5
0      seconds
.lp +5 5
1      minutes
.lp +5 5
2      hours
.lp +5 5
3      day of the month (1-31)
.lp +5 5
4      month (0-11)
.lp +5 5
5      year \*- 1900
.lp +5 5
6      day of the week (Sunday = 0)
.lp +5 5

While this may have been an original decision by Dennis Ritchie, it's also possible it was copied from an even earlier system. Does anyone know?

Comment via: facebook, lesswrong, hacker news, substack

Recent posts on blogs I like:

On Apologizing To Kids

Everyone is so weird about apologizing to children.

via Thing of Things August 25, 2025

Against the Teapot Hold in Contra Dancing

The teapot hold is the most dangerous common contra dancing figure, so I’ve been avoiding it. The teapot hold, sometimes called a "courtesy turn hold,” requires one dancer to connect with their hand behind their back. When I realized I could avoid put…

via Emma Azelborn August 25, 2025

Little Puppy

She's very little and she likes to do stuff with me. She also likes to bark around and run around and jump around. She also likes to go to places with me and that's all I have.

via Nora Wise's Blog Posts August 23, 2025

more     (via openring)