code: public static Date parseHttpDate(CharSequence txt, int start, int end) {
          int length = end - start;
          if (length == 0) {
              return null;
          } else if (length < 0) {
              throw new IllegalArgumentException("Can't have end < start");
          } else if (length > 64) {
              throw new IllegalArgumentException("Can't parse more than 64 chars," +
                      "looks like a user error or a malformed header");
          }
          return formatter().parse0(checkNotNull(txt, "txt"), start, end);
      }
docs: Parse some text into a {@link Date}, according to RFC6265
  @param txt text to parse
  @param start the start index inside {@code txt}
  @param end the end index inside {@code txt}
  @return a {@link Date}, or null if text couldn't be parsed