When iHeart Radio Music Festival organizers cut Green Day's set short on Friday night (Sept. 21), frontman Billie Joe Armstrong was far from happy about it. He went on an expletive-fueled rant onstage ... and now he's entering rehab.

A rep for Green Day confirmed that Armstrong is entering rehab for substance abuse (they didn't specify which substance or substances those are) and told TMZ, "Green Day wants to apologize to those they offended at the iHeartRadio Festival in Las Vegas."

Armstrong clearly felt disrespected by organizers for not only cutting his band's set short, but also for letting them be the last to know about it. It appears he doesn't quite believe a lot of the other acts have paid their dues yet, and it really didn't sit well with him.

The punk staples were set to perform for 45 minutes, but were cut to 30 to accommodate sets by Usher (whose performance ran an extra 25 minutes) and Rihanna, who performed like she didn't even want to be there. What's worse? No one told Armstrong until he was already performing and had one minute left. He stopped performing during their classic 'Basketcase' and basically flipped out.

"F--- this shit," Armstrong ranted. "I'm gonna play a new song. F--- this s---. Give me a f---ing break, one minute left!" he spat. "One minute f---ing left. You're gonna give me one minute? Look at that f---ing sign right there," he told the crowd. "One minute. Let me tell you something," he said. "Let me f---ing tell you something. I've been around since 19-eighty-f---ing-eight," he said to raucous cheers. "And you're gonna give me one f---ing minute? You gotta be f---ing kidding me! What the f---? I'm not f---ing Justin Bieber, you motherf---ers! You gotta be f---ing joking. This is a f---ing joke," he added furiously. "I got one minute left? And now I got nothin' left. Let me show you what one minute f---ing means."

Armstrong then smashed his guitar onstage, flipped off the cameras, and roared, "One f---ing minute. God love you all," he said to the crowd. "We'll be back."

It sounds like the audience was on Armstrong's side: After the band left the stage, the crowd chanted, "Green Day! Green Day! Green Day!" While we wish Billie Joe a speedy recovery, we also hope he doesn't lose his rebellious spirit. Ironically, by cutting off one of the few rock bands on the festival's bill, the iHeart Radio Festival may have achieved its only true rock n' roll moment.