Harry and Taylor met at the 2012 Kids' Choice Awards on March 31, 2012. They began texting each other back then, but it seems that nothing actually happened between the duo at that time.
Over the summer, Harry hung out with a couple of other girls (including Cara Delevingne) while Taylor spent some time with Conor Kennedy.
After Taylor's relationship with Conor ended, it appears that she began a relationship with Harry rather quickly, as they were seen out after the VMA's on September 6, 2012.
Their relationship went public when she began wearing his paper airplane necklace while out in Paris in early November. Their first public date was at the Central Park there and that spiraled into a crazy December of outings in New York City, London, and Utah (where they got into a snowmobile accident). They even spent New Year's Eve together and publicly brought in the new year with a kiss at Times Square.
After New Year's Eve, they went on vacation together to the British Virgin Islands, however this was cut short after they supposedly got into a massive argument and broke up, leading to Taylor leaving the islands alone.
For most of 2013, their relationship seemed to be sour with Taylor mocking Harry at the Grammy's and then again at the VMA's, however, after the VMA's, they attended an after party together with Ed Sheeran.
In 2014, it seems that they have resolved their issues and are good friends, or at least amicable. In late 2014, Taylor released her album "1989" with a load of songs about Harry, all relatively good in spirit. These songs included "Style", "Out of The Woods" (in which Taylor references the necklace and the snowmobile incident), "All You Had to do Was Stay", "Wildest Dreams", "I Know Places" and "Clean" among others.
They were seen together at Caleb Followill's birthday party as well as an after party for the Victoria's Secret Fashion Show. Harry was even heard whistling after Taylor received an award at the 2014 AMA's.
In early 2015, Taylor released her music video for her single about Harry called "Style". In this music video, she is shown holding Harry's paper airplane necklace numerous times.
Swift began a relationship with DJ Calvin Harris later in March 2015, after 2 years and a half single.
In April 2017, Harry publicly talked about his relationship with Swift for the very first time during an interview for Rolling Stone:
"When I see photos from that day," he says, "I think: Relationships are hard, at any age. And adding in that you don't really understand exactly how it works when you're 18, trying to navigate all that stuff didn't make it easier. I mean, you're a little bit awkward to begin with. You're on a date with someone you really like. It should be that simple, right? It was a learning experience for sure. But at the heart of it – I just wanted it to be a normal date."
He's well aware that at least two of Swift's songs – "Out of the Woods" and "Style" – are considered to be about their romance. ("You've got that long hair slicked back, white T-shirt," she sang in "Style.") "I mean, I don't know if they're about me or not ..." he says, attempting gallant discretion, "but the issue is, she's so good, they're bloody everywhere." He smiles. "I write from my experiences; everyone does that. I'm lucky if everything [we went through] helped create those songs. That's what hits your heart. That's the stuff that's hardest to say, and it's the stuff I talk least about. That's the part that's about the two people. I'm never going to tell anybody everything." (Fans wondered whether "Perfect," a song Styles co-wrote for One Direction, might have been about Swift: "And if you like cameras flashing every time we go out/And if you're looking for someone to write your breakup songs about/Baby, I'm perfect.")
Was he able to tell her that he admired the songs? "Yes and no," he says after a long pause. "She doesn't need me to tell her they're great. They're great songs ... It's the most amazing unspoken dialogue ever."
Is there anything he'd want to say to Swift today? "Maybe this is where you write down that I left!" He laughs, and looks off. "I don't know," he finally says. "Certain things don't work out. There's a lot of things that can be right, and it's still wrong. In writing songs about stuff like that, I like tipping a hat to the time together. You're celebrating the fact it was powerful and made you feel something, rather than 'this didn't work out, and that's bad.' And if you run into that person, maybe it's awkward, maybe you have to get drunk ... but you shared something. Meeting someone new, sharing those experiences, it's the best shit ever. So thank you."
Styles supposedly wrote "Two Ghosts" as a response to Swift's "Style" - "Two Ghosts" is the fourth track on Styles's self-titled debut album released on May 12, 2017.