public static boolean isDateMaybe(byte[] buf, int offset, int len) {
    // maybe valid - too expensive to check without a parse
    return len >= 8;
  }
returns false, when the bytes definitely cannot be parsed into a date/timestamp.

Y2k requirements and dash requirements say the string has to be at least
yyyy-m-m = 8 bytes or more minimum; Timestamp needs to be at least 1 byte longer,
but the Date check is necessary, but not sufficient.
serde/src/java/org/apache/hadoop/hive/serde2/lazy/LazyUtils.java