Academy upgrades.
For the newbies.
Plan when you make your U21 upgrade, it takes 28 days from completion of the upgrade to provide two players.
The players will be 18yo, so plan your upgrade so that the players join you at the earliest the first day of the following season to the start of your upgrade.
Why?.
Simple!, if your player arrives at the end of the season in just a few days he will be 19yo and his whole development with have lost a full season, he will never catch that season up without buying skill points.
The same applies to further upgrades to the youth system, the upgrade takes the same time but your second upgrade will see the players arrive a few days earlier in 24 days.
If you have not upgraded your academy directly as the previous one supplied the players any players already being developed will be binned when you start the next upgrade, so always check before you start the upgrade.