Happy Birthday, Dearest Deceased

On 15th December 2009, Kageyama Tobio went missing and his body was found 7 days later on his birthday. Murdered.
Now, 1 minute before 22 December 2017, Oikawa Tooru was having a toast to celebrate Kageyama's supposed 20th birthday when he found himself waking up on his last year as a middle schooler.
And 1 month before Kageyama Tobio's supposed murder.