This story is being written purely for fun. I'm not receiving any financial gains from the writing of this.
Harry Potter was dying.
At the age of 112, he would have normally had about 60 more years of life left in him; wizards lived much longer than muggles, after all. But Harry had known that he would die younger than most for quite a while. You see, in Harry's second year at Hogwarts School of Witchcraft and Wizardry, the basilisk bite that he'd received had been healed by the tears of a phoenix.
At 60, Harry Potter had planned on living a long time with his three children, James Sirius Potter, Albus Severus Potter, and Lily Luna Potter and his two grandchildren. But that all changed on Harry's 35th birthday, with the arrival of his animagus form.
Harry had become a phoenix animagus on his 35th birthday (it was said to have been good luck to complete a transformation on said day). What he hadn't realized was that phoenixes could only be healed by phoenixes in the most dire of circumstances; when a phoenix is healed by a phoenix, it takes away their powers of immortality and forces them to die 100 years to the day. Harry had, in effect, signed his own death sentence by becoming an animagus.
When Harry realized this (that is, as soon as Hermione had told him), he decided that he had to do something else worthwhile in his lifetime. He had fought Death Eaters, (spoiler alert) killed Voldemort, and had married, and had three children. But life had become boring. Every day was now similar to him. Harry abruptly changed his course; he began paying Hermione to tutor him in charms and potions, and soon began working on a cure for werewolves. He specifically chose the topic because he realized that it would take him time, and time was what Harry needed to expend. He wanted to work on something enjoyable, something that benefited others right to his dying day.
Many years later, when Harry had found a cure for the werewolves, Harry was content for a number of years. But the death of one Severus Snape still haunted him; he had died after giving up everything to serve the light, and, by association, his enemy's son. There was a time when Harry had simply revered him, but that had passed after the naming of his second son. Now, though, it was resurfacing differently – Harry wanted the man to have a second chance at life.
"I am, after all, the master of Death," Harry had thought, smiling. But then his grin faltered. "What would Ginny say…" he mumbled. "Don't be getting too big for your britches, Harry Potter," he imitated her shrill voice, laughing. Even after 70 years of scoldings, Harry still found Ginny's gentile admonishments both sweet and funny.
As Harry had already decided that he'd wanted to spend the rest of his life researching, Severus Snape became Harry's new obsession, much worse, even, than Hermione's with S.P.E.W.
From the age of 70 to the age of 107, Harry's family barely saw him. He was always in the library working on time magic and trying to figure out how to fix his problem. In Harry's 94th year, he finally found a potion and a spell that, when combined, enabled time travel. At the age of 107, he found a spell to send someone to a copy of his dimension when applied with that potion and other spell.
Because Harry barely saw his family, he spent his 107th through 112th years completely with his family. He effectively became the family babysitter for his now numerous great grandchildren and a constant companion of his wife, in-laws, children, nieces, nephews, and friends. Every spare second was used up.
As Harry approached his soon-to-be death day, he thought of a new plan. If he left early enough, he could first send Snape off and then go back in time to his second year, place a glamour on his animagus form so that he looked like Fawkes, hold Fawkes back, and do everything that Fawkes had done. Harry didn't think that it was fatal for a phoenix to heal itself, but he didn't know for sure. He did, however, suppose that it was worth a try.
Harry left to tearful goodbyes a week before his possible death day. He first changed into his animagus form and then downed the potion that separated his soul from his body and placed it in a container made of air. Only a wind phoenix animagus could use the potion because they had to harden the air themselves. He then cast the ante mortem Severī spell of his own invention and landed lightly next to his younger self. He had luckily placed his invisibility cloak on his spirit self before he left. As soon as his younger self left, Harry flew above the Professor in his animagus form and cried tears on him. He then changed back and forced him to drink a blood replenishing potion. Next he cast a sleeping spell on him.
Harry then apparated Snape to a muggle park. He wasn't too worried about the statue of secrecy at a time like this, but luckily they weren't seen. Next, Harry cast Enervate, episkey, and scourgify on Snape at once.
Snape then woke up. "Where am I?" he mumbled. Harry then knew that he must be feeling awful; Professor Snape detested mumbling. Harry then poured pepper-up potion down his throat. He sat up immediately.
"Who are you?" Snape asked, getting ready to draw his wand.
"I," Harry replied, "am Harry James Potter."
Snape then went to draw his wand and cast stupefy, but was left with an empty hand and no effect. When Harry then passed Snape his wand, Snape seemed more willing to listen.
"How can you be Potter? You're 100 some years old."
"Well, Professor, I ended up inventing a new type of magic; I figured out how to use time magic."
"You invented time magic?"
"Yes," Harry replied. "Your second worst brewer invented time magic, created two spells, and created a potion. Not to mention the fact that I eventually improved your Wolfsbane and created a cure for lycanthropy."
"I feel faint," Snape said dryly.
"You shouldn't," Harry said, concerned. "My phoenix tears should have healed you completely."
"For the record, Potter, I can joke. Wait, YOUR phoenix tears? How ever did you manage to buy such an expensive thing?" Snape asked.
"I'll let you in on a secret, Professor," Harry said. He then changed into his animagus form.
That was precisely when Snape fainted.
And Harry laughed.
Snape stood up again. "You shall never mention this again, Potter," he growled.
"Certainly, Professor," Harry said without smiling, but his shining eyes betrayed him.
"But really, Professor, I have to tell you why I came back now, for I have to leave in only hours.
"Severus," Snape said.
"Excuse me?" Harry asked bewildered.
"You have earned the right to call me Severus," he replied, grimacing.
"Well then, Sev," Harry said smirking, "You…" Harry was then cut off.
"NEVER call me Sev," Snape yelled at the top of his lungs attracting the attention of two muggles. He repeated it more quietly. "Sev," he said, not facing Harry, "was you mother's name for me until the end of our fifth year. I fully expect NEVER to hear it again." He had since then turned back and was now looking angry.
"Sorry, Sir, but I can't agree to that," Harry said, and he did so quite cheekily for a post-centenarian.
"What?" Snape asked dangerously, twisting back around.
"You see," Harry said, "I came back here to give you a better life. I'm going to send you to a dimension that is the same as ours except that it has only progressed to a day before your fifth year OWLs. I figured that you might want to get perfect Os. You will then severe ties with Avery and Mulciber and the rest of the aspiring Death Eaters and won't call my mum a mudblood. You will then hopefully marry her; my dad got his chance in this dimension, you get yours in the next."
Snape was struck speechless.
"What's in it for you, Potter?" he asked.
And then he fainted.
Harry laughed and was very thankful that he'd never have to answer that question.