SEOUL, South Korea — South Korea said on Monday that it would begin this week to dismantle the loudspeakers that have broadcast propaganda across the border into North Korea for decades, the first step toward implementing an agreement the country’s leaders reached during their summit meeting on Friday.

South Korea’s Defense Ministry made the announcement about the loudspeakers shortly after North Korea’s legislative body, the Supreme People’s Assembly, adopted a decree to synchronize the North’s time zone with the South.

The North adopted “Pyongyang Time” in 2015, which put its clocks 30 minutes behind the South. Beginning on Thursday, North Korea will be on the same time as the South, nine hours ahead of Coordinated Universal Time, the North said through its official Korean Central News Agency.

South Korea said it would start on Tuesday to dismantle its network of propaganda loudspeakers along the 155-mile-long border, pulling back one of its most potent weapons of Cold War-era psychological warfare.