ddev_theme_dev/web/core/misc/timezone.js

/**
 * @file
 * Timezone detection.
 */

(function ($, Drupal) {
  /**
   * Set the client's system time zone as default values of form fields.
   *
   * @type {Drupal~behavior}
   */
  Drupal.behaviors.setTimezone = {
    attach(context, settings) {
      const timezone = once('timezone', '.timezone-detect', context);
      if (timezone.length) {
        const tz = new Intl.DateTimeFormat().resolvedOptions().timeZone;
        // Ensure that the timezone value returned by the browser is supported
        // by the server.
        if (tz && $(timezone).find(`option[value="${tz}"]`).length) {
          timezone.forEach((item) => {
            item.value = tz;
          });
          return;
        }

        const dateString = Date();
        // In some client environments, date strings include a time zone
        // abbreviation, between 3 and 5 letters enclosed in parentheses,
        // which can be interpreted by PHP.
        const matches = dateString.match(/\(([A-Z]{3,5})\)/);
        const abbreviation = matches ? matches[1] : 0;

        // For all other client environments, the abbreviation is set to "0"
        // and the current offset from UTC and daylight saving time status are
        // used to guess the time zone.
        const dateNow = new Date();
        const offsetNow = dateNow.getTimezoneOffset() * -60;

        // Use January 1 and July 1 as test dates for determining daylight
        // saving time status by comparing their offsets.
        const dateJan = new Date(dateNow.getFullYear(), 0, 1, 12, 0, 0, 0);
        const dateJul = new Date(dateNow.getFullYear(), 6, 1, 12, 0, 0, 0);
        const offsetJan = dateJan.getTimezoneOffset() * -60;
        const offsetJul = dateJul.getTimezoneOffset() * -60;

        let isDaylightSavingTime;
        // If the offset from UTC is identical on January 1 and July 1,
        // assume daylight saving time is not used in this time zone.
        if (offsetJan === offsetJul) {
          isDaylightSavingTime = '';
        }
        // If the maximum annual offset is equivalent to the current offset,
        // assume daylight saving time is in effect.
        else if (Math.max(offsetJan, offsetJul) === offsetNow) {
          isDaylightSavingTime = 1;
        }
        // Otherwise, assume daylight saving time is not in effect.
        else {
          isDaylightSavingTime = 0;
        }

        // Submit request to the system/timezone callback and set the form
        // field to the response time zone. The client date is passed to the
        // callback for debugging purposes. Submit a synchronous request to
        // avoid database errors associated with concurrent requests
        // during install.
        const path = `system/timezone/${abbreviation}/${offsetNow}/${isDaylightSavingTime}`;
        $.ajax({
          async: false,
          url: Drupal.url(path),
          dataType: 'json',
          success(data) {
            if (data) {
              document.querySelectorAll('.timezone-detect').forEach((item) => {
                item.value = data;
              });
            }
          },
        });
      }
    },
  };
})(jQuery, Drupal);