code: func (c *Client) signBlob(ctx context.Context, bb schema.Buildable, sigTime time.Time) (string, error) {
  	signer, err := c.Signer()
  	if err != nil {
  		return "", err
  	}
  	return bb.Builder().SignAt(ctx, signer, sigTime)
  }
docs: // sigTime optionally specifies the signature time.
  // If zero, the current time is used.