Allow anonymous users to use a SESSION variable to set timezone

Description of the need

In Timezone Detect, I'd like to allow the timezone to be detected and set in a SESSION variable for anonymous users, so website dates/times display in the timezone they are in. However, as noted here, we'd need some core changes to allow us to use the SESSION variable for anonymous timezones.

Is there a way we can update backdrop_get_user_timezone() to allow this?