The date shown on the chart of 9 June is a mistake, since the execution date of Buckbeak the Hippogriff, which is also the date for the events in Chapters 16-22 of Harry Potter and the Prisoner of Azkaban, is set in the text as “sixth of June”.
Cornelius Fudge, the Minister for Magic, reads the official notice of execution in the presence of Rubeus Hagrid, Albus Dumbledore, an unnamed member of the Committee for the Disposal of Dangerous Creatures and the executioner, Walden Macnair:
“It is the decision of the Committee for the Disposal of Dangerous Creatures that the Hippogriff Buckbeak, hereafter called the condemned, shall be executed on the sixth of June at sundown –” (PA21)
The essay "Time Line: The End of PA" by Hollydaze contains further timeline notes for 6th June.
From the Web
Writing by J K Rowling on WizardingWorld (Pottermore): Time-Turner
Harry Potter Wiki: Time-Turner