Different result in parsing datetime via calendsr or simpleDateFormatter

This is due to the difference between the Julian and Gregorian calendar systems.

SimpleDateFormat uses the default calendar system, which I believe to be GregorianCalendar on both your system and mine. GregorianCalendar (despite its name) switches between the Gregorian calendar system and the Julian calendar system, based on the gregorianChange property. It assumes that any date provided after that cut-over is Gregorian, and any before it is Julian. The default cut-over is in 1582.

DatatypeConverter uses a pure proleptic Gregorian calendar instead, as that's what the W3C XML Schema docs require.

That means if you parse a value just before the calendar switch, you'll see a large difference - and that difference will get smaller as you go further back in time, with a difference of 3 days every 400 years. (The three century-years which aren't divisible by 400, and so are leap years in the Julian calendar but not in the Gregorian calendar.)

If you set the calendar in the SimpleDateFormat to a GregorianCalendar which you've called setGregorianChange(Long.MIN_VALUE) on first, the two will agree.

Here's code to make it easier to explore the difference:

import javax.xml.bind.DatatypeConverter;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.concurrent.TimeUnit;

public class Test {    
    public static void main(String[] args) throws ParseException {
        convert("0001-01-01T00:00:00");
        convert("1000-01-01T00:00:00");
        convert("1580-01-01T00:00:00");
        convert("1590-01-01T00:00:00");
        convert("1980-03-01T00:00:00");
    }

    private static void convert(String input) throws ParseException {
        Calendar datatypeConverterResult = DatatypeConverter.parseDateTime(input);
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
        Date sdfResult = simpleDateFormat.parse(input);

        System.out.println("Input: " + input);
        long datatypeConverterMillis = datatypeConverterResult.getTimeInMillis();
        long sdfResultMillis = sdfResult.getTime();
        long days = TimeUnit.MILLISECONDS.toDays(datatypeConverterMillis - sdfResultMillis);
        System.out.println("DatatypeConverter epoch millis: " + datatypeConverterMillis);
        System.out.println("SimpleDateTime epoch millis: " + sdfResultMillis);
        System.out.println("Difference in days: " + days);
        System.out.println("Parsed calendar time zone: " + datatypeConverterResult.getTimeZone().getID());
        System.out.println();
    }
}

Note that on Java 9, you need to specify the module explicitly. This is simplest done with java.se.ee:

$ javac Test.java --add-modules java.se.ee
$ java --add-modules java.se.ee Test

Output on my box:

Input: 0001-01-01T00:00:00
DatatypeConverter epoch millis: -62135596800000
SimpleDateTime epoch millis: -62135769600000
Difference in days: 2
Parsed calendar time zone: Europe/London

Input: 1000-01-01T00:00:00
DatatypeConverter epoch millis: -30610224000000
SimpleDateTime epoch millis: -30609792000000
Difference in days: -5
Parsed calendar time zone: Europe/London

Input: 1580-01-01T00:00:00
DatatypeConverter epoch millis: -12307248000000
SimpleDateTime epoch millis: -12306384000000
Difference in days: -10
Parsed calendar time zone: Europe/London

Input: 1590-01-01T00:00:00
DatatypeConverter epoch millis: -11991628800000
SimpleDateTime epoch millis: -11991628800000
Difference in days: 0
Parsed calendar time zone: Europe/London

Input: 1980-03-01T00:00:00
DatatypeConverter epoch millis: 320716800000
SimpleDateTime epoch millis: 320716800000
Difference in days: 0
Parsed calendar time zone: Europe/London