code: func AppendHTTPDate(dst []byte, date time.Time) []byte {
  	dst = date.In(time.UTC).AppendFormat(dst, time.RFC1123)
  	copy(dst[len(dst)-3:], strGMT)
  	return dst
  }
docs: // AppendHTTPDate appends HTTP-compliant (RFC1123) representation of date
  // to dst and returns the extended dst.