A passage from https://howardhinnant.github.io/date/tz.html that may bear investigation appears below.

    @dashdashado

You can also customize the build by defining macros (e.g. on the command line) as follows...

AUTO_DOWNLOAD

If AUTO_DOWNLOAD is 1 then first access to the timezone database will install it if it hasn't been installed, and if it has, will use the remote API to install the latest version if not already installed.