Guru Gobind Singh (1666-1708 C.E.) was the tenth and the last Guru of the Sikhs. According to Julian calendar, he was born on 22nd December 1666 in Patna of Bihar. Presently Julian calendar does not exist and is replaced by Gregorian calendar.
As per Gregorian calendar, Guru Gobind Singh was born on 1st January 1667. Whether it is Julian calendar or Gregorian calendar, the Hindu date of birth of Guru Gobind Singh falls on the same day. As per Hindu calendar Guru Gobind Singh was born on Saptami, Paush, Shukla Paksha, in 1723 Vikram Samvat. There is no controversy with the date of birth of Guru Gobind Singh as often found with other Gurus and Saints.
The recently developed Nanakshahi calendar fixed the Guru Gobind Singh's Jayanti on 6th January and later it is revised to 5th January