A country having its own time zone is not unheard of. In fact, even individual cities used to set their own clocks. But by the late 19th century, railroads in the U.S. had started making travel easier. Differing local times led to confusion for passengers and operators. So, in 1883, U.S. and Canadian railroads adopted a standardized system of time zones.

One year later, officials at a global conference in Washington, D.C., created a worldwide system of 24 standard time zones: one for each hour of the day. They designated the prime meridian (0° longitude) as the starting point for the zones. That imaginary line runs through Greenwich, England—a part of London. The time at the prime meridian is known as Coordinated Universal Time (UTC).

While most countries follow the standard time zone system, a few have reset their clocks for various reasons. In 2011, Samoa changed its time to make communication with nearby New Zealand easier. And in 2015, North Korea turned its clocks 30 minutes behind those of Japan to mark the 70th anniversary of Korea’s liberation from Japanese occupation.

Regardless of why a country changes its time zone, the move can still be frustrating for residents.

“It’s hard to adjust,” Gao Li, a sanitation worker in western China told reporters. “I often think we must be the only people who eat dinner at midnight.”