threetenbp: Date formatting fails on some Android devices

I haven’t been able to reproduce this myself, but I am getting reports from the wild that sometimes LocalDate.format() is returning an invalid result on some Android devices. (Using https://github.com/JakeWharton/ThreeTenABP.)

Here’s what happens, as confirmed by the log call in this method.

• On entry to this method, month is 2016-08.
• startOfMonth is correctly set to 2016-08-01.
• monthString gets set to 0000-00-00 which is wrong.

private Observable<Void> ensureDataForMonthExistsInternal(YearMonth month) {
    final LocalDate startOfMonth = month.atDay(1);
    final String    monthString  = startOfMonth.format(DateTimeFormatter.ISO_LOCAL_DATE);
    Timber.i("Calling ensureDataForMonthExists with month %s, as date %s, formatted %s", month, startOfMonth, monthString);

So far the issue seems to affect only Motorola devices running Android 6.0.

Does threetenbp actually use possibly-broken system APIs for date formatting? Or is it completely self-contained? Intuitively, my expectation is that using DateTimeFormatter.ISO_LOCAL_DATE should not require the locale or any other information to be read from the system.

This was originally reported as https://github.com/JakeWharton/ThreeTenABP/issues/33.