According to state media, at 23:30 local time (15:00 GMT) on Friday the country’s clocks moved forward 30 minutes to midnight.
The reset is “the first practical step” to speed up Korean unification, the official KCNA news agency said.
Meanwhile, US President Donald Trump said he has a date for his meeting with the North’s leader, Kim Jong-Un.
Mr Trump will host South Korea’s president Moon Jae-in at the White House on 22 May to discuss the upcoming meeting.
Until now, it was on Pyongyang Time, which is half an hour behind South Korea and Japan.
It introduced the extra 30 minutes in 2015 as a stand against “wicked Japanese imperialists”, because its hours were changed to match Tokyo’s when the Korean peninsula was under Japan’s rule.
South Korea (which is nine hours ahead of GMT) had also moved away from Japanese time in the 1950s but it switched back again in the 1960s.
Two clocks – showing the time in North and South Korea – sit side by side in Panmunjom, the town in the so-called Demilitarised Zone (DMZ) where the two countries’ leaders recently met.
After the talks, South Korea’s presidency sent a tweet saying Mr Kim was distressed by seeing the clocks.