Function sub_datetime_promoted 

pub fn sub_datetime_promoted<D>(
    a: &Array<DateTime64, D>,
    unit_a: TimeUnit,
    b: &Array<DateTime64, D>,
    unit_b: TimeUnit,
) -> Result<(Array<Timedelta64, D>, TimeUnit), FerrayError>
where
    D: Dimension,

Element-wise datetime[unit_a] - datetime[unit_b] → timedelta[finer].

Both operands are rescaled to TimeUnit::finer(unit_a, unit_b) before the per-element subtraction. Returns (result, target_unit).

Errors

Returns FerrayError::ShapeMismatch if the shapes differ.