Puratan Janamsakhi is the most contemporary and probably authentic. It was written in the early 1600s making to a very reliable source. Bahi Vir Singh writes it is the oldest janamsakhi though he goes with a mid 1600s date of Guru Hargobinds time.
He says the Hindalias have made their own corrupt version of the janamsakhi. He also says that Bhai Bala Wala Janamsakhi is a copy of this while changing parts to fit the agenda of the author who Bhai Vir Singh believes to be the Hindalias.
Bhai Vir Singh believe that Sakhis of this Janamsakhi were each written at different times. The third sakhi is written a while after the Mughals while Sakhis about Guru Arjan Dev Jis shabads were, by the language, written during Guru Hargobind Jis time. Another Sakhis appears to be of Guru Gobind Singhs time.
The language is south western Punjabi and it does not seem the writer is a professional nor an expert. There are many errors that seem to be copying mistakes.
The first Sakhi says Guru Nanak was born in the month of Vaisakh in the third period of the moonlight night. I.e amrit vela. It interestingly says that by the time he was 5 Hindus and Muslims started to consider him and avatar or a saint.
There is a lot of Gurbani used in this Janamsakhi. The author makes the mistake of attributing Saloks of the fifth Guru to Guru Nanak.