Calvin Harris is a DJ and Producer who dated Taylor in 2015-2016 for 15 months. When together, their relationship was slightly public- mostly on Instagram and a bit of twitter. As both Calvin and Taylor are song writers, during their relationship, they ended up writing the hit song "This Is What You Came For" that Rihanna sings and Calvin produced. Fun fact: Taylor's vocals are actually used in place of some of Rihannas in the post-chorus riffs. Although she is not credited for them in song credits, Calvin did come out and say explicitly that her vocals are in the song.
Calvin is also apart of the Bleachella Love Trio with Tom Hiddleston. For more information on that, visit the Bleachella page.
During the Reputation era Taylor dated Tom Hiddleston after Calvin Harris and before Joe Alwyn. Taylor has expressed how she felt “trapped” in her relationship with one of her ex’s (we assume Calvin, because of the length of their relationship), and how she wanted to escape. She took the “getaway car” to Tom, and then later left him for Joe
Taylor mentioned that she felt trapped in a relationship during the time of her 1989 era, which we can assume is with Calvin. “Best believe I’m still bejeweled when I walk in the room, I can still make the whole place shimmer” points at her threatening her leaving because she has so many more options and could/can be happier
During the 1989 era Taylor dated Calvin for around a year, before getting with Tom Hiddleston relatively fast, which did spark rumors of possible infidelity, especially because of the nasty way things went down between Calvin and Taylor. This is her explaining that story