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  • 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, hacker news

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