ce01c175df
DateField seeded the picker with ZoneId.systemDefault() but converted
the selectedDateMillis back with ZoneId.of("UTC"). For any non-UTC
user, the field and the picker displayed different days, and
confirming without re-picking silently shifted the export range by a
day. Use UTC on both sides (the DatePickerState contract is that
selectedDateMillis is UTC midnight of the picked day).