What evidence does the text provide to establish the historical age of the Brihad Guru Charitra?

The text uses a specific historical reference to date the Brihad Guru Charitra. Chapter 0 explains that its author was Saraswati Gangadhar, who wrote it under the command of Shri Samarth Sadguru Narsimha Saraswati Maharaj. To establish its age, the text then cites a different work, Shri Dasganu Maharaj's 'Shri Bhaktalilamrut'. In the 30th chapter of that book, it is written that Narsimha Saraswati Maharaj reincarnated and later took Samadhi in Shake 1807. From this external reference, the source concludes that the Guru Charitra must have been written 'much before Shake 1807,' thereby confirming its antiquity.