Function add_business_days 

pub fn add_business_days(z: i64, n: i64) -> i64

Advance a day number by n business days (skipping weekends): n > 0 moves forward, n < 0 backward. Landing rule is “next business day in the direction of travel”, so add_business_days(friday, 1) is the following Monday and add_business_days(monday, -1) is the preceding Friday.