code: func (bb *Builder) ModTime() (t time.Time, ok bool) {
  	s, ok := bb.m["unixMtime"].(string)
  	if !ok {
  		return
  	}
  	t, err := time.Parse(time.RFC3339, s)
  	if err != nil {
  		return
  	}
  	return t, true
  }
docs: // ModTime returns the "unixMtime" modtime field, if set.