Function spot_effective_date 

pub fn spot_effective_date(
    cal: &Arc<dyn Calendar>,
    valuation: NaiveDate,
    spot_lag: Period,
    bdc: BusinessDayConvention,
) -> NaiveDate

The template-convention spot (default effective) date: valuation advanced by spot_lag on cal — the one seam both the product builders and the calibration helpers resolve a swap’s effective date through, so a helper and a Product from the same template always describe the same instrument. A Day-unit lag advances in business days (a zero-day lag rolls forward to the next business day on or after valuation); other units advance by calendar arithmetic under bdc.