Ignoring the specific example given, JavaScript Date objects use zero-based months depending on how you construct or interact with them, so I'm assuming the OP was thinking of a zero-based example and just wrote it out using 08 in the string.
months sometimes start at zero in programming languages. days of the week, as well, often.
there is also the problem that in some locales the first day of the week is thought to be Sunday rather than Monday, so regardless of 0-indexing or not, we can't even agree on that IRL :-D