When Ethan Renner (Kevin Costner) tries to get the French police to remove the family of immigrant squatters from his little-used Paris apartment, they tell him he must wait until April 13, when winter ends. And when he threatens to do it himself, they tell him he'll get arrested. There is, in fact, a law in France that protects squatters who occupy a residence during winter for more than two days. The owner can't remove them except by filing a lawsuit.
When Ethan (Kevin Costner) enters a tattoo parlor, one can see the nude back of a girl with a very mexican "Lady of Guadalupe/Holy Death" tattoo. This mystery girl is also seen tattooing Mr. Jang (Min-sik Choi) in the movie Lucy (2014). Both films were written by Luc Besson. The tattoed model is uncredited in both films.
The check for $150,000 that Ethan (Kevin Costner) receives for his services is curiously dated, November 22, 2013, the 50th anniversary of John F. Kennedy's assassination, a crime Costner (as D.A. Jim Garrison) investigated in JFK (1991).
The car Vivi Delay (Amber Heard) picks Ethan Renner (Kevin Costner) up in is a Peugot RCZ and retails for around $36,000. According to Peugot, actress Heard liked the car so much she wanted to perform her own stunts.
Ethan Renner (Kevin Costner) is seen rescuing Zooey (Hailee Steinfeld) from a group of boys in the washroom then proceeds to carry her out of the nightclub. In The Bodyguard (1992), Costner's character rescues Whitney Houston from her fans on the stage and also ends up carrying her out of the nightclub.
The movie's storyline refers to a "dirty bomb", which is a radiological dispersal device (RDD), the Wikipedia website defining it as "a speculative radiological weapon that combines radioactive material with conventional explosives. The purpose of the weapon is to contaminate the area around the dispersal agent/conventional explosion with radioactive material, serving primarily as an area denial device against civilians".
When the student is talking to Ethan Renner (Kevin Costner) after he is picking up Zooey (Hailee Steinfeld) at school, the student makes a cowboy reference. Costner played western lawman Wyatt Earp in Wyatt Earp (1994), a cowboy in _Open Range_, had the lead role in the western Dances with Wolves (1990), and also portrayed a cowboy in Silverado (1985), one of his earliest starring roles.
The trivia items below may give away important plot points.
The name of the terminal highly invasive glioma brain cancer that Ethan Renner (Kevin Costner) had was "glioblastoma" (GBM). The American Brain Tumour Association states: "Glioblastomas (GBM) are tumors that arise from astrocytes-the star-shaped cells that make up the 'glue-like', or supportive tissue of the brain. These tumors are usually highly malignant (cancerous) because the cells reproduce quickly and they are supported by a large network of blood vessels".
Ethan's handler is Vivi Delay. Vivi is short for Vivien or Vivian, depending on gender. The name derives from a Latin term meaning "alive" and thus ironically has a double meaning in the context of the movie. First, it applies to Ethan's not truly living, seeking out his wife and daughter for a close relationship, until he learns he is dying; his life has been "delayed." Second, the name applies to the experimental serum Vivi gives him as part of his compensation for this final mission-- a serum which may indeed keep him alive. Significantly, earlier in the movie, he doubts he'll live until Christmas; in the closing scene, he's celebrating Christmas with his wife and daughter, suggesting that he is being healed.
The meaning and relevance of the film's title is that it refers to the estimate number of days that Ethan Renner (Kevin Costner) has to complete his mission prior to facing death from terminal brain cancer.