My name
is
Jon Skeet

Why in order to compare hijri with iso dates you need to wrap a DateTime in LocalDateTime? (Joda)

I am trying to convert an ISO (Gregorian) date into a hijri (Islamic) date and the compare the first to a hijri date.

What I do and it does not work is:

    Chronology iso = ISOChronology.getInstanceUTC();
    Chronology hijri = IslamicChronology.getInstanceUTC();

    DateTime dtISO = new DateTime("2015-03-12T10:04:43", iso);

    System.out.println("dtISO: " + dtISO);
    DateTime toHijri = new DateTime(dtISO, hijri);
    System.out.println("toHijri: " + toHijri); // 1436-05-21T10:04:43.000Z
    DateTimeFormatter formatter = DateTimeFormat.forPattern("YYYY-MM-dd HH-mm");

    DateTime hijriDt = new DateTime(formatter.parseDateTime("1442-05-21 10-10"), iso);

    if (hijriDt.isAfter(toHijri)) {
        System.out.println("hijriDt date is after toHijri date");
    }
    if (hijriDt.isBefore(toHijri)) {
        System.out.println("hijriDt date is before toHijri date");
    }
    if (hijriDt.isEqual(toHijri)) {
        System.out.println("hijriDt date is equal toHijri date");
    }

No matter what the hijri date is it always returns "hijriDt date is before toHijri date".

Now as I was experimenting I tried this:

    Chronology iso = ISOChronology.getInstanceUTC();
    Chronology hijri = IslamicChronology.getInstanceUTC();

    LocalDateTime toHijri = new LocalDateTime(new DateTime("2015-03-12T10:04:43", iso), hijri);
    System.out.println("toHijri: " + toHijri); // 1436-05-21T10:04:43.000Z
    DateTimeFormatter formatter = DateTimeFormat.forPattern("YYYY-MM-dd HH-mm");

    LocalDateTime hijriDt = new LocalDateTime(formatter.parseDateTime("1442-05-21 10-10"), iso);

    if (hijriDt.isAfter(toHijri)) {
        System.out.println("hijriDt date is after toHijri date");
    }
    if (hijriDt.isBefore(toHijri)) {
        System.out.println("hijriDt date is before toHijri date");
    }
    if (hijriDt.isEqual(toHijri)) {
        System.out.println("hijriDt date is equal toHijri date");
    }

which works like a charm. Notice how the toHijri date is constructed: LocalDateTime toHijri = new LocalDateTime(new DateTime("2015-03-12T10:04:43", iso), hijri);

A DateTime instance is wrapped inside a LocalDateTime.

Why is this happening? Why the first block does not work?

This is not what you want:

DateTime hijriDt = new DateTime(formatter.parseDateTime("1442-05-21 10-10"), iso);

That's not using the Hijri calendar anywhere:

Your formatter is using ISO (by default)
You're parsing a date in year 1442 ISO, which isn't what you want
You're then "converting" that into the ISO calendar (which it already is)
Your variable name suggests that you're expecting it to be in the Hijri calendar, but it's not

I suspect you want:

DateTimeFormatter formatter = DateTimeFormat.forPattern("yyyy-MM-dd HH-mm")
    .withChronology(hijri);
DateTime hijriDt = formatter.parseDateTime("1442-05-21 10-10");

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