Ruby's Date object has a MONTHNAMES constant that returns an array of names
of the month. You'd think that means the array contains 12 items. However, the
size of that array is 13.
> Date::MONTHNAMES
=> [nil, "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]Notice it has all 12 months, plus an initial value of nil.
This is because it allows us to more intuitive access a month by it's index
without having to do a little subtraction. If I want to know what the 9th month
is, I can do an array access for 9.
> Date::MONTHNAMES[9]
=> "September"Because arrays in Ruby use 0-based indexing, without this baked in nil value,
you'd instead get "October" when passing in 9.