Determining the string formats for datetime-timezone

Question

When loading a .log file I get the message

Warning (emacs): Failed to determine system timezone; consider customizing ‘datetime-timezone’ variable

Going to the customization menu, the options are not evident:

Hide Datetime Timezone: nil
    State : STANDARD.
   Default timezone for date-time formatting and parsing. Hide
   Leave unset to let the library auto-determine it from your OS
   when necessary.
Groups: Datetime

Lines that also appear in the source

(defcustom datetime-timezone nil
  "Default timezone for date-time formatting and parsing.
Leave unset to let the library auto-determine it from your OS
when necessary."
  :group 'datetime
  :type  'symbol)

1. What is the appropriate way to set my timezone on macOS so that datetime-timezone would inherit it without a duplication in ~/.emacs.
2. Failing that, what is the format for this customization? (For some reason, C-h i does not include a menu for datetime in my installation.)

Answer 1

According to the documentation it should be determined with the TZ environment variable :

The default time zone is determined by the TZ environment variable. [...] For example, you can tell Emacs to default to Universal Time with (setenv "TZ" "UTC0"). If TZ is not in the environment, Emacs uses system wall clock time, which is a platform-dependent default time zone.

Have you tried to set this variable ?

You should be able to set it from Emacs, using eval-expression (M-:) :

(setenv "TZ" "America/New_York")

or you can set it at launch before running emacs command from your shell :

$ TZ="America/New_York" emacs

Tested successfully on Linux, but can't tell for Mac OS.