Why JavaScript counts month from zero (0)?

One possible explanation:

They might've considered months to be an enumeration (first index being 0) and days not since they don't have a name associated with them.

Or rather, they thought the number of the day was the actual representation of the day (the same way months are represented as numbers in a date like 12/31), as if you could make a enumeration with numbers as the variables, but actually 0-based.

So actually, for the months, perhaps they thought the proper enumeration representation would be to use the month's name, instead of numbers, and they would've done the same if days had a name representation. Imagine if we'd say January Five, January Sixth, instead of January 5, January 6, etc., then perhaps they'd have made a 0-based enumeration for days too...

Perhaps subconsciously they thought about an enumeration for months as {January, February, ...} and for days as {One, Two, Three, ...}, except for days you access the day as a number rather than the name, like 1 for One, etc., so impossible to start at 0...

http://stackoverflow.com/questions/2552483/why-does-the-month-argument-range-from-0-to-11-in-javascripts-date-constructor