Joel promised Ellie he would teach her to play guitar when they were done with their mission, a promise that he kept, although this wasn't shown until Season 2, Episode 6.

Joel Miller's story In The Last of Us is full of tragedy, difficult decisions, ... Joel still learned how to play the guitar. Joel's daughter, Sarah, was born on July 20, ...

The first flashback featured Joel gifting Ellie a guitar for her 15th birthday. She asked him to play something, which was when he played "Future Days" from Pearl Jam's 2013 album Lightning Bolt.