Mumbai: Author Chetan Bhagat says it's a working birthday for actor Arjun Kapoor, who turned 31 on Sunday. Arjun is currently busy shooting for "Half Girlfriend", a film based on Bhagat's novel of the same name. "Working birthday for Arjun Kapoor as he braves the Delhi heat for 'Half Girlfriend'. On set, cake cutting pics," Bhagat tweeted and also posted a few images in which the actor is seen cutting a basketball-shaped cake on the set. The film, directed by Mohit Suri, also features Shraddha Kapoor. Scheduled for a worldwide release on April 28 next year, the movie will include songs sung by Rishi Rich and Shraddha herself.