Your concern is very much relatable Gopal Prabhu.
Ideally, Guru Maharaja’s Vyasa-puja should be celebrated on his Janma-tithi, according to the Vedic calendar.
However, the practical difficulty is that while Guru Maharaja’s date of birth is known,his exact time of birth is not confirmed. Guru Maharaja himself mentioned in one of his lectures that he is not sure of the time.
Because of this uncertainty, it is not possible at present to accurately determine the Janma-tithi. Therefore, for simplicity and consistency, we are currently following the Gregorian calendar date as his appearance day.
Your servant.