On 18 September, Stalin dispatched General H. M. Zakharov to Korea to advise Kim Il-sung to halt his offensive around the Pusan perimeter and to redeploy his forces to defend Seoul. Chinese commanders were not briefed on North Korean troop numbers or operational plans. As the overall commander of Chinese forces, Zhou Enlai suggested that the North Koreans should attempt to eliminate the enemy forces at Inchon only if they had reserves of at least 100,000 men; otherwise, he advised the North Koreans to withdraw their forces north.
Try to answer this question if possible (otherwise reply "unanswerable"): When was it advised for North Koreans to attempt to defeat the opposing forces at Inchon?
if they had reserves of at least 100,000 men