Task: Question Understanding
Input (English): Passage: The defeat at Ngasaunggyan broke the back of Burmese defenses. The Burmese army lost several thousand men as well as senior commanders. Kaungsin, the next fort inline, fell just six days later on 9 December 1283. The Mongol sources say that the Burmese lost 10,000 men at Kaungsin. The Mongol armies pushed farther south into the Irrawaddy valley. They took the ancient Burmese capital of Tagaung, about 380 km north of Pagan on 5 February 1284. There, the invaders paused their advance. They found the heat of the searing Irrawaddy valley excessive, and evacuated Tagaung, allowing the Burmese to return to Tagaung on 10 May 1284. But the Mongol army renewed their offensive in the following dry season. They retook Tagaung, and defeated another Burmese stand south of Tagaung, probably near Hanlin, on 26 January 1285, opening the way to Pagan, about 270 km south. After the defeat, the king panicked, and fled to Lower Burma. The evacuation proved premature. The Mongol forces did not advance on Pagan as it was not part of their invasion plan. The country fell into chaos. In Lower Burma, the king found himself isolated, let alone plan a counterattack. Although his sons ruled the key Lower Burma ports , the king did not trust any of them, and he and his court settled at Hlegya, west of Prome. Without the full support of his sons, the presence of the king and his small army impressed no one. The governor of Pegu revolted that same year. The king managed to send two small expeditions to Pegu but they both failed. Now, the entire eastern half of Lower Burma  was in open revolt.
Question: On what date did the Burmese army lose several thousand men as well as senior commanders?
Output (English): date