From the Middle Ages till present days, the “Biography of Sultan Jalal al-Din Mankburny” by his secretary Muhammad an-Nasavi is considered to be one the most important Arab-Persian sources about the Mongol conquest of Central Asia and Iran. Eyewitness of the Mongol invasion, an-Nasavi provides a good deal of valuable information which are not found in other synchronous historical works. Despite the fact that this historical source has been introduced into scientific circulation for a long time and is well known to the researchers, there are still some questions about the information contained in it. This article is devoted to the question of double dating of the same events in the “Biography”, which has not been the object of special study. For the battle on the Turgay plain between the troops of Khorezmshah Ala ad-Din Muhammad and the Mongolian corps under the command of Juchi Khan, an-Nasavi cites two different dates: 616 and 612 AH, and the battle near Isfahan between Jalal al-Din’s troops and Mongols he dates as 624 and 625 AH. Nowadays, I propose a preliminary hypothesis on the origin of these contradictions. In the both cases, an-Nasavi wanted to correct the faults of his contemporary Ibn al-Athir whose work he cited in his own. In the first case, an-Nasavi may have confused two Mongolian campaigns about which he could have heard only rumors: against the Merkites (in 1215/16) and against Kuchlug Khan (in 1218). By contrast, the later Arab-Persian historians don’t confuse these campaigns, being familiar with Chinese and Mongolian sources. And in the second case, Ibn al-Athir erroneously cites two battles near Isfahan (in 1226/27 and in 1228). An-Nasavi, as he knew this event much better, wanted to correct this fault. But having corrected the date in the text, he may have forgotten to correct it in the chapter’s head where he had already copied the wrong date from Ibn al-Athir’s work.