It's not madness to mix horse racing and music - Horse Racing - Telegraph
Does it mean when nine months are out (at the end of nine months), one would need to book a band?The risks are the weather and you need to book a band nine months out.
No.
It means you need to book a band nine months in advance.
Rover
I couldn't find an entry in many dictionaries relating to this use of "out". Could my example be slangy?
Does this mean "book something X months out" is informal/nonstandard English?
For many of us there is a subtle difference between 'not standard usage' and 'non-standard usage', and a bigger difference between 'not common' and 'uncommon'. You seem determined to read into our words what you want to read.
"book something X months out" could be an Americanism....
"He fell out the window." = "He fell through the window."
From that, "out" = "through". Then "book something X months out." could mean "book something through X months."![]()