"Let's go see a movie Saturday afternoon." "I need this done by Thursday." All these statements are common and completely understandable by everyone. Change to

"Let's go see a movie Saturday afternoon." "I need this done by Thursday." All these statements are common and completely understandable by everyone. Change to UTC, and the statements are ambiguous. It will confusing for so many people, with no obvious benefit. China is fine, because the day doesn't change during daylight. Yeah, computers should use UTC. It's easy for them to convert to/from local time for the user. This is what computers are good at.