One of the more distinctive features of a Quentin Tarantino film is the soundtrack.  Starting with Reservoir Dogs (1992), Tarantino has largely relied on pre-existing songs to fill his films.  While making Jackie Brown (1997), producer Harvey Weinstein told Tarantino the film needed a score.  Instead of creating an original score for the film, Tarantino used the score from the blaxploitation film Coffy (1973), which also stars Pam Grier.  Tarantino uses pre-existing music so as not to relinquish creative control to a composer.  In Django Unchained (2012) however, Tarantino did include music written specifically for the film.  And the music is noticeably contemporary.  Songs like John Legend’s “Who Did That to You?” and Rick Ross’s “100 Black Coffins” are not part of a traditional western soundtrack.  It is unorthodox for a film set during pre-Civil War America like Django Unchained to include songs by twentieth century artists like Johnny Cash and Anthony Hamilton.

Tarantino explained his rationale for using modern music in a 19th century period piece to Charles McGrath in an interview with The New York Times.

CG: “I have to say at first it was a little jarring to hear hip-hop in Django.  You’re sitting there and think you’re in one kind of musical universe, and then you break the frame.”

QT: “I don’t think like that.  If it’s score, it’s my decision.  I can put anything on it I want.  If in Inglourious Basterds (2009) Shosanna turned on the radio and David Bowie started coming out, that would be weird and anachronistic… But if it’s a score, it’s score—it can be anything.  I just have to pull it off.”

In other words, when it comes to non-diegetic music, chronology is unimportant because the song is for the cinema audience and not the film's characters.  The music is meant to compliment the film.

For instance, the Jim Croce song “I’ve Got a Name” was released over one hundred years after the movie takes place and yet the music and lyrics are perfect for the scene.  The Croce ballad plays after Django (Jamie Foxx) has killed his old tormentors the Brittle Brothers.  He is a partner with bounty hunter King Schultz (Christoph Waltz), and as he suits up to hunt down more bad guys, we realize that Django has transformed and is no longer a slave, but rather a freeman

Moments before the film's climax, Django escapes from a group of Australian slavers and rides back to Candyland to the tune of John Legend's “Who Did That to You?”.  “Now I’m not afraid to do the Lord’s work / You say vengeance is His, but I’m a-do it first.”  The music and the lyrics work perfectly and it is hard to imagine how another piece of music could achieve the same desired effect and the music is not anachronistic because Django does not hear the music, only the audience does.