code: func (s Session) GetExpiration(expireAfterDuration time.Duration) time.Time {
  	if expireAfterDuration == 0 || s[TimestampKey] == SessionValueName {
  		// Expire after closing browser
  		return time.Time{}
  	}
  	return time.Now().Add(expireAfterDuration)
  }
docs: // getExpiration return a time.Time with the session's expiration date.
  // It uses the passed in expireAfterDuration to add with the current time if the timeout is not
  // browser dependent (ie session). If previous session has set to "session", the time returned is time.IsZero()