"It is often helpful to know the time of the last activity associated with a entity. The canonical usage is to discover when a disconnected user last accessed its server. The 'jabber:iq:last' namespace provides a method for retrieving that information. The 'jabber:iq:last' namespace can also be used to discover or publicize when a connected user was last active on the server (i.e., the user's idle time) or to query servers and components about their current uptime."
There it is, just the extension.
Application needs to take care in answering the possible request using this extension, thus this as far as the library goes.