said in ICU-20710,
> It looks like we can workaround around this issue by checking if any UFIELD_CATEGORY_DATE_INTERVAL_SPAN fields are present in the UFormattedValue. And when no UFIELD_CATEGORY_DATE_INTERVAL_SPAN are present, we simply format the start date using a UDateFormat instead of using UDateIntervalFormat.
We should add a test for this behavior: UFIELD_CATEGORY_DATE_INTERVAL_SPAN should be present in the UFormattedValue if and only if a date interval was formatted. If the field is not present, then that should imply that no date interval was formatted.