UTC is an acronym for Univeral Time Coordinated. You can think of it as GMT with leap seconds. It’s the international reference time. It’s always the same time UTC all over the planet.
Japan doesn’t observe Daylight Saving Time, so the local time there (JST in English) is always 9 hours ahead of UTC. So currently, local time in Japan is 13 hours ahead of US Eastern Daylight Time.
The Soviets used a different time system so it would be harder for us to convert during wartime when things, especially nukes, were moving fast.