Function day_groups 

pub fn day_groups(
    entries: &[CatalogEntry],
    rows: &[usize],
    key: DateKey,
    clock: &Clock,
) -> Vec<DayGroup>

Split already-sorted rows into day groups by key (use Sort::date_key). Rows sorted by something else just yield more, smaller groups — callers should skip headers when date_key is None.