Add further reading references to 'tz' argument of excel_numeric_to_date

[sfirke]

Related to #232, I think we should provide a reference link in the documentation for the tz argument that would allow users to read more about valid timezones and see a list of them. I found my way to https://data.iana.org/time-zones/tz-link.html but would like something simpler. A list of common timezones on a website might be more useful to the typical user than shell commands to download a tarball.

Maybe two links, the one I have above for further / more advanced reading and a simpler one with list of common timezones?

[billdenney]

I have a similar response here that unless we want to handle the complexity of timezones internally (which I think is better left to other functions and packages), we would be better off linking to the functions that handle the timezones for us.

To get the list, we can suggest base::OlsonNames(), and to get the current time zone, we can suggest base::Sys.timezone(). That would be my preference.

[sfirke]

I like your fix: add to the documentation of the tz argument that they can see a list of timezones with base::OlsonNames() and their current timezone with base::Sys.timezone().