Why subtract 1 from months in JavaScript when converting to a date object? [duplicate]

Answer 1

Months represented in JavaScript Date objects are 0-based. That is, January is represented as 0, February as 1, March as 2, etc.

For some interesting answers as to why this behaviour is implemented across many languages, see Zero-based month numbering.

"the combination of zero-based arrays and 1-based month values does bring a problem. To get the month name of month 9, you would have to get item 8 from the month array. Some developers would be happy with decreasing the month number before getting it's name. Others preferred to change the month into something zero-based since people just want to know the name, not the number. It's a personal view."

Answer 2

I think the technical reason it does this would be because month is an enumerated field, meaning that it can be mapped to an array of month names, and arrays count from zero, whereas years and days are not enumerated; you always just want the actual number value, so that's what is stored.

It is a perfectly valid question to ask though, because if you're using months as a number then it can seem a bit weird (not to mention easy to forget).

By the way, if you're working with dates, particularly extracting dates from strings and formatting them into strings, you might want to look into Date.js, which is a javascript library that can help a lot with all this kind of thing.