Function turn_dates 

pub(crate) fn turn_dates(
    period: &str,
    start: NaiveDate,
    end: NaiveDate,
) -> PyResult<Vec<NaiveDate>>

The turn-of-period dates after start and up to end: every 31 December for "YearEnd", every quarter end for "QuarterEnd" — the dates a step curve’s turn knots land on.