THE meeting between the leaders of North and South Korea, Kim Jong Un and Moon Jae-in, on April 27th was widely considered a success. The goodwill flowed; the two men visited both sides of the border; a peace tree was planted. The actual, useful results were more limited. But one immediate consequence will be felt from May 5th. On that day, North Korea’s clocks will be turned forward by half an hour so that both Koreas sit in the same time zone again, nine hours ahead of GMT. It was only in August 2015 that North Korea’s clocks were turned back by 30 minutes. Mr Kim ordered that shift ostensibly to reverse the imposition of Tokyo time by “wicked Japanese imperialists” in 1912. (South Korea did the same thing in 1954, but switched back to Japanese time in 1961 to encourage trade.) Mr Kim was apparently moved to undo the change after seeing two clocks (pictured) on his recent hop into South Korea showing, side by side, the different times in Seoul and Pyongyang.

North Korea’s time-travelling is the latest example of a long historical tradition of rulers making political points by adjusting clocks and calendars. Doing so alters a fundamental aspect of daily life, literally at a stroke. What better illustration could there be of a ruler’s might than control over time itself? Admittedly, not all such changes stand the test of time: French revolutionaries, keen to emphasise the break with their monarchist past, failed to get their ten-hour clock and entirely new calendar to stick after imposing them in 1793. The Soviet Union’s experiments with five- and six-day weeks during the 1930s also failed to endure. But those changes that do persist can memorialise past rulers more effectively than any physical monument. July was named in honour of Julius Caesar in 45 BC, and August was later renamed after Augustus Caesar.

In the modern era, control of time provides a way to underline the clout of central government: both India and China, despite their size, have a single time zone, which keeps everyone marching in step with the capital. It also offers an opportunity for emphasising independence and non-conformity. Hugo Chávez turned the clocks back by half an hour in 2007 to move Venezuela into its own time zone—supposedly to allow a “fairer distribution of the sunrise” but also to ensure that the socialist republic did not have to share a time zone with its arch-enemy, the United States.

In theory, modern technology offers liberation from temporal tyranny. Smartphones and computers can translate between time zones and calendar systems, allowing people to use whichever they like. In practice, however, time zones and calendars are more than just arbitrary ways to rule lines on time. They do not merely specify how to refer to a particular instant or period; they also dictate and co-ordinate activities across entire societies, in particular by defining which days are working days and national holidays. These have to be consistent within countries and, in some cases, between them: just ask Saudi Arabia, which in 2013 moved its weekend from Thursday-Friday to Friday-Saturday, to bring it into line with other Arab states. The need for such co-ordination means there is no escape from centralised control of clocks and calendars—which explains, in turn, why the tendency to tinker with them for political purposes is timeless.