Probably because we like round numbers. But there are half-hour aligned time-zones. Newfoundland is -3.5. It's currently 11:20 am here in Toronto and 12:50 p

Probably because we like round numbers. But there are half-hour aligned time-zones. Newfoundland is -3.5. It's currently 11:20 am here in Toronto and 12:50 pm in St. John's. Then there's the Chatham Islands of New Zealand with a quarter hour offset at UTC +12:45. There are also regions where two bordering time zones change by more than 1 hour. At the Pakistani-Chinese border you gain or lose three hours.