Java 8 LocalDateTime is parsing invalid date

You just need a strict ResolverStyle.

Parsing a text string occurs in two phases. Phase 1 is a basic text parse according to the fields added to the builder. Phase 2 resolves the parsed field-value pairs into date and/or time objects. This style is used to control how phase 2, resolving, happens.

Sample code - where withResolverStyle(ResolverStyle.STRICT) is the important change, along with the use of uuuu rather than yyyy (where uuuu is "year" and "yyyy" is "year of era", and therefore ambiguous):

import java.time.*;
import java.time.format.*;
import java.util.*;

public class Test {

    public static void main(String[] args) {
        String dateFormat = "HH:mm:ss MM/dd/uuuu";
        String dateString = "11:30:59 02/31/2015";
        DateTimeFormatter dateTimeFormatter = DateTimeFormatter
            .ofPattern(dateFormat, Locale.US)
            .withResolverStyle(ResolverStyle.STRICT);
        try {
            LocalDateTime date = LocalDateTime.parse(dateString, dateTimeFormatter);
            System.out.println(date);
        } catch (DateTimeParseException e) {
            // Throw invalid date message
            System.out.println("Exception was thrown");
        }
    }
}

It is not rounding down. February has never had 31 days, and it is impossible to use a validating date / time object to represent a day that doesn't exist.

As a result, it takes the invalid input and gives you the best approximation to the correct date (the last date of February that year).

SimpleDateFormat inherits from DateFormat which has a setLenient(boolean value) method on it. I would expect that if you called setLenient(true) prior to parsing, it would probably complain more, as detailed in the javadocs.

The Java 8 DateTimeFormatter uses yyyy to mean YEAR_OF_ERA, and uuuu to mean YEAR. You need to modify your pattern string as follows:

String dateFormat = "HH:mm:ss MM/dd/uuuu";

The DateTimeFormatter defaults to using the SMART resolver style, but you want it to use the STRICT resolver style. Modify your dateTimeFormatter initialization code as follows:

DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(dateFormat, Locale.US)
                                                       .withResolverStyle(ResolverStyle.STRICT);