code: func (s *Signer) SignJSON(ctx context.Context, json string, t time.Time) (string, error) {
  	sr := s.baseSigReq
  	sr.UnsignedJSON = json
  	sr.SignatureTime = t
  	return sr.Sign(ctx)
  }
docs: // SignJSON signs the provided json at the optional time t.
  // If t is the zero Time, the current time is used.