In the nine-second clip, which shows a chocolate cake frosted with the words "1 year" and two red hearts, Harris excitedly says, "One year, we got one-year cake!" as the "New Romantics" singer smiles and nods.
A few hours later, Swift posted an Instagram photo of a gold locket engraved with the date "3.6.15."
"One year down!" the 26-year-old captioned it, along with a heart emoji.