1999
There had been so much death. The war had left so many devastated on both sides. While Harry had known that it was impossible for everyone to walk away unscathed, he had not expected the result of fighting Voldemort to be so catastrophic. The Wizarding World had been utterly devastated and would take decades to recover, assuming it was even given the chance to.
That was decades that shouldn’t have been needed had it not been for Dumbledore’s arrogance. The man had sent abused children back to dangerous homes – Tom Riddle, Severus Snape, himself – and labeled some of them dark (and therefore evil in his eyes) when they did whatever they could to escape the situation, Riddle especially, or used their background to easily manipulate them to his purposes – Harry had seen this in himself, as well as many other realizations, once he was free of the bloody soul piece that was influencing him. All of which had only increased prejudices. Granted, the man’s every move was in his attempt to save the Wizarding World, but he made decisions about school children with the mindset of a General; he weighed their lives over the rest of the Wizarding World and found the “greater good” to be worth the price.
But the cost had been too much. There was just too much death. Families on both sides have lost loved ones and destruction of their world looked to be following. The year he spent helping the rebuilding efforts had been just as destructive due to the prejudices remaining from the war. The light has remained bigoted against anything they considered dark, while the dark has reacted violently in response to the light. The devastation has only gotten worse since the war ended, and magic has become more unstable due to the lack of balance between the two sides of magic. Death had only followed more death.
With that in mind, Harry made his way to the time turners in the Department of Mysteries, where he was now holding a time turner that would send him back years rather than hours. The question now was when. While he believed Dumbledore’s actions (and bigoted views) had a hand in creating Voldemort, Harry didn’t believe there was anything that could be done to stop Tom Riddle; there was too much prejudices and too much hate in the boy. Prejudice issue aside, Harry wouldn’t be able to commit infanticide, no matter who it was, which didn’t even take all the unknowns that would occur due to such an action into account. However, what Harry could do, was destroy the Horcruxes before the prophesy was spoken and then deal with Voldemort from there.
He knows the ring was made in 1942 with Riddle murdering his father and was placed by the time the diadem was made and placed in 1967 after Riddle was refused for the DADA Professor position; the diary was made with Myrtle’s death in 1943 and the cup in 1956 with Hepzibah Smith’s death, both of which he could only presume were given to Lucius and Bellatrix after they’d proven themselves loyal sometime in the 70s; the locket was made around the same time as the cup and stashed in the cave in 1979 just before Regulus Black defected and stole it; and, finally, he becomes one in 1981 with Nagini following sometime after that.
The diadem is the first guaranteed placement he knew in 1967 with the ring not too far behind it. Considering that it would easier to get to the diadem if he had an established identity before then, Harry decided that he would travel to 1965, settle into his new life in the past, and then get to work fixing the future. Decision made, the eighteen-year-old turned the time turner and vanished from 1999.