"This is all wrong. Albus? Can you hear me? All this for a frigging Time-Turner."
-- Scorpius Malfoy (CC1.19)
With only Scorpius Malfoy left outside the Weaponised Library, he works under pressure to solve the third and final riddle. This releases Delphi and Albus Potter from inside the bookshelves and reveals the Experimental Time-Turner hidden inside a book (CC1.19).
This is the day after Albus and Scorpius escape from the Hogwarts Express and visit the St Oswald's Home - so during 2 September 2020.
Scorpius Malfoy knows about libraries and recognises the banned books contained in Hermione Granger's "Weaponised Library". He is very surprised when it then attacks them: "I opened a book. Something which has - in all my years on this planet - never been a particularly dangerous activity" (CC1.19).
From the Web
Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 31 July 2016, with Esther Smith in the role of Delphi, Anthony Boyle as Scorpius Malfoy and Sam Clemmett as Albus Potter: Harry Potter and the Cursed Child, Parts One and Two