Every user stores the session-data in the database (xxx_sessions).
The tricky thing here is a custom data stored in the session with
$_SESSION['CURRENT_DOC_YEAR'] = $current_doc_year;.
This session data is populated at login from a global variable controlled from administrators.
In the administrative interface, we can use
variable_set(ACTIVE_YEAR_KEY, $year); to set another year. So, when users log out and in again, they will see the updated value. Apparently, users don't log out and in so often.
How can forcefully update all the active sessions without truncating the sessions table (and throw all users out)?
Unfortunately, the theme uses
$_SESSION['CURRENT_DOC_YEAR'] to show some links and texts at various places of the site.