Which of the following is grammatically correct?
a) In 1952, John joined the army.
b) In 1952 John joined the army.
Is the comma needed?
Which of the following is grammatically correct?
a) In 1952, John joined the army.
b) In 1952 John joined the army.
Is the comma needed?
Because "In 1952" is an introductory prepositional phrase, you may use a comma after the end of it and before the rest of the sentence. According to Purdue OWL, a comma is recommended for longer prepositional phrases. Either choice is technically correct.
https://owl.purdue.edu/owl/general_writing/punctuation/commas/commas_after_introductions.html