you misunderstood it or their wording is not the best. Once construction is complete, the countdown starts (28 days, 24 days, 20 days whatever level it is at). That's why it's so important for players to understand to NOT build until the day they receive young players because the clock resets itself. Example, if your players are supposed to arrive in 8 days and you start an upgrade, the clock will reset after it's done to 24, 20, 16 or 12 days.