get_expiry_age()
Returns the number of seconds until this session expires. For sessions with no custom expiration (or those set to expire at browser close), this will equal SESSION_COOKIE_AGE.
This function accepts two optional keyword arguments:
modification: last modification of the session, as a datetime object. Defaults to the current time.
expiry: expiry information for the session, as a datetime object, an int (in seconds), or None. Defaults to the value stored in the session by set_