Section 27. Seniority of officers of the same grade shall be determined by their date of rank, which shall be their date of appointment. Between officers of the same grade and date of rank the relative rank shall be determined by considering dates of rank in lower grades in the armed forces of the commonwealth, commissioned, warrant and enlisted, until seniority is established. If seniority is not so established it will be decided by relative age. The day of the appointment of an officer shall be stated in his commission and shall be considered as the date thereof.