Get timezone offset from the browser and use it for timezones in Rails

application.js

// jquery-cookies retired, moved this to js-cookie which no longer requires jQuery
import Cookies from 'js-cookie'

Cookies.set('tz', (new Date()).getTimezoneOffset());

application_controller.rb

class ApplicationController < ActionController::Base
  before_filter :set_timezone
  
  # …
private

  def set_timezone
    # if the user has set their time_zone we default to that
    # if there is no user.time_zone set, then we default to the browser time_zone
    if current_user
      if current_user.time_zone
        Time.zone = current_user.time_zone
        logger.info("Using current_user.time_zone #{current_user.time_zone}")
      elsif browser_timezone
        Time.zone = browser_timezone
        logger.info("Using browser_timezone since current_user does not have time_zone set")
      end
    elsif browser_timezone
      Time.zone = browser_timezone
      logger.info("Not logged in, so using browser_timezone")
    end
  end
  
  def browser_timezone
    @browser_timezone ||= begin
      ActiveSupport::TimeZone[-cookies[:tz].to_i.minutes]
    end if cookies[:tz].present?
  end

end