Do I need to put months on my resume?
While including the months during which you held a particular job is not strictly necessary, it can provide a more complete picture of your work history for potential employers. Including both the start and end months for each position can give a clearer idea of how long you spent in each role and how your career has progressed over time.
That being said, if you prefer to only list the years you held a position, that is still acceptable. The important thing is to be consistent and to provide accurate information. The goal of your resume is to effectively communicate your qualifications and work experience to potential employers, so choose the format that you think best serves that purpose.