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History of Zero-Based Months?

52 points| expjpi | 3 years ago |jefftk.com

28 comments

order

gerikson|3 years ago

At least in Perl, the rationale for this (apart from copying C) is that it's very easy to reference a list (array) of month names if the month indices are zero-based.

DonaldFisk|3 years ago

How difficult is it to either subtract 1 before indexing your array, or have a 13 element array with a dummy value in element 0? Deprecate time.h and adopt ISO 8601 already.

nayuki|3 years ago

Zero-based months is awkward when printing as a number, but convenient when indexing an array:

    const MONTHS = ["Jan", "Feb", "Mar", ..., "Dec"];
    console.log(MONTHS[d.getMonth()]);

stingraycharles|3 years ago

Is subtracting 1 really that much an inconvenience, given how awkward they are?

pvorb|3 years ago

Yes, that was always my theory for why months are 0-based. Back in the days of early Unix, this might have had a tiny performance benefit that was enough for them to choose that implementation.

lofatdairy|3 years ago

The same justification exists in JS (where it's copied from Java which copies it from C). However, interestingly checking the FORTRAN IV specification documents for the PDP-10, it's not implemented as a 3 letter ASCII abbreviation. That document dates to 1975, I don't know if I can find if the date exists in the first version of the document, which should date to 1967. I was unable to find a reference to a builtin in the base FORTRAN II, which only provides 20 builtin functions, date not making the list. I think newer versions of FORTRAN77 has idate which is 1-indexed but I couldn't find it in the older standard listed on wg5's specification documents.

[^1]: https://github.com/PDP-10/f40/tree/master/doc

photochemsyn|3 years ago

I think it all comes down to whether you're counting elements as the primary goal. However, months are poorly defined relative to something like an astronomical year or a standard day, so they're more like objects. That is to say, adding different months doesn't give you the same number of days as a result.

Hence, dealing with months is perhaps a bit more like indexing variable-sized objects, and for that purpose traditionally, the array-of-pointers approach uses zero-based pointer arithmetic, which is what the designers might have been thinking.

Really however, the months should probably be treated more like structs, with the number of days in that month being a data member. This would allow sanity checks, i.e. entering Feb 30 should raise an error, but not May 30.

_OOps_|3 years ago

I think it's important to be able to work with mod(%) 12 for some operations on year <--> month relationship. All years have 12 months. For day of the month you need additional logic to handle diferent lengths... and other issues.

zzo38computer|3 years ago

This reason makes sense. Note that different uses will have different useful conventions, and converting between them may be necessary. (Although this is true of more than only month numbering.)