Change it from...

$mon is the month itself, in the range 0..11 with 0 indicating January and 11 indicating December.

...to...

$mon is the month offset, in the range 0..11 with 0 indicating January and 11 indicating December.

...as the former implies that $mon is the number of the month which people might assume starts at 1 (and stop reading). But my guess is that anyone who has a problem hasn't read the current docs. (but, hey at least they don't just assume tm_year is the last two digits nowadays).