UDateIntervalFormat doesn't provide a way to change the Gregorian change date and instead always uses the default Gregorian change date from icu::GregorianCalendar.
This issue is blocking Mozilla from implementing the ECMA-402 proposal <https://github.com/tc39/proposal-intl-DateTimeFormat-formatRange>. (Firefox code uses the ICU C API instead of the C++ API to get a stable ABI, because we allow embedders to provide their own ICU version.)
Expected: Prints 10/04/1582.
Actual: Prints 09/24/1582.
And in comparison with the C++ API: