Tutorial doesn't convert the string id in ZSH

[Simon Frid]

In the first step of the tutorial, the PROJECT_ID errors out when attempting to divide the datestring by a number in zsh.

$ PROJECT_ID="vault-$(($(date +%s%N)/1000000))"
zsh: bad math expression: operator expected at `N/1000000'

For context

$ zsh --version
zsh 5.3 (x86_64-apple-darwin17.0)