title: How can a client make use of expires_in?
body: <p>Observing OAuth2, <code>expires_in</code> seems to be a common parameter returned alongside an access token. The value of <code>expires_in</code> is the number of seconds remaining until the access token expires. I'm having trouble seeing how client applications would be able to make use of this in a convenient way. Expecting clients to count down the seconds after receiving an access token seems annoying at best. Wouldn't it be easier to set something like <code>expires_at</code> with a future timestamp when it expires?</p>