code: func NewSigner(issuer, responder *x509.Certificate, key crypto.Signer, interval time.Duration) (Signer, error) {
  	return &StandardSigner{
  		issuer:    issuer,
  		responder: responder,
  		key:       key,
  		interval:  interval,
  	}, nil
  }
docs: // NewSigner simply constructs a new StandardSigner object from the inputs,
  // taking the interval in seconds