golang / gorilla：更新会话到期？

Question

I know how to use gorilla under golang to manage sessions. But what I'm trying to accomplish is to optionally set the session expiry time to a later date at run time, depending upon various application conditions. I haven't been able to figure out how to update this expiry time.

Consider the following code fragment ...

skey  := "some sort of secret key"
sname := "some sort of session name"

session_store := sessions.NewCookieStore([]byte(skey))
session_store.Options = &sessions.Options{                                                                                        
    MaxAge: 300,                                                                                                       
}

// `r` is previously defined as the current *http.Request
sess, err := session_store.Get(r, sname)

As written, sess will expire 300 seconds after it was initialized. But how can I extend the lifetime of sess before this much time passes, so that its expiry will then occur at a later time?

Thank you in advance.