Before proceeding, it is probably a good idea to reconsider the use of the word “genocide” in the question. While North Korea’s government has been among the most brutal in the world since its inception, and while the North Korean government’s policies have directly contributed to the deaths by starvation of millions of its people, the word “genocide” carries certain connotations more properly associated with deliberate efforts on the part of a government or regime to physically eliminate through death and expulsion an entire category of people, for example, Europe’s Jews during the 1940s, Cambodian intellectuals and middle class families during the rule of the Khmer Rouge, and the atrocities imposed upon the Tutsi minority in Rwanda in 1994 at the hands of the country’s majority Hutu. What has transpired in North Korea is deserving of world condemnation, and has resulted in more losses of lives than in Rwanda or, possibly, Cambodia, the political dynamics and causes of massive numbers of deaths are considerably more complicated.
That said, one could logically date the period of atrocities against the Korean people residing north of the 38th Parallel from the formation of the Democratic Peoples’ Republic of Korea (DPRK, or North Korea) in 1950 to the present day. The Japanese Imperial Army had occupied the Korean Peninsula from 1910 until its defeat in World War II in 1945. The peninsula was then divided between the Soviet-occupied north and the U.S.-occupied south. From its inception to the present, North Korea has been ruled by the family of its first leader, Kim il-Sung, whose death in 1994 was followed by his son, Kim Jong-il’s period of despotic rule. Kim Jong-il’s death in 2011 was followed by the rise of his son, Kim Jong-un. Each member of the family – the only Communist dynasty in history – has ruled with an iron fist, with even the slightest hint or suggestion of dissent from Communist Party orthodoxy met with imprisonment not just of the “offending” citizen but of his entire family under extraordinarily harsh conditions in the country’s system of concentration camps. Many suspected of dissent – a category that includes failure to display the leader’s portrait in one’s home – are executed.
To the extent the concept of “genocide” can be applied, it would involve the regime’s economic policies, which have condemned the population to a near-feudal existence outside of the capital city of Pyongyang. Primitive agricultural practices, exacerbated by occasional flooding seasons that destroy crops, have resulted in the deaths of millions of North Koreans and the gradual but steady diminishment of the entire nation’s capacity for physical growth. In other words, entire generations of North Koreans are experiencing stunted growth because of the lack of nutrition in their meager diets.
The simple answer to the question, then, is 1945 to the present.