Function datetime_plus_duration 

pub fn datetime_plus_duration(
    date: i64,
    nanos_of_day: i64,
    dur: &DurationValue,
) -> (i64, i64)

localdatetime/datetime + duration (#920): add the signed months, then days, then the sub-day time (carrying whole-day overflow into the date). Returns the resulting (date, nanos_of_day).