Everyone keeps correcting my baby’s age

My baby was born December 13, 2024 (on a Friday ) I count Friday to Friday as 1 week . Making him 12 weeks this past Friday ! March 7, 2025. Everyone one around me can’t seem to grasp that concept. I keep getting told he will be 12 weeks on March 13th, 2025. How should I be keeping track of my baby’s age !?!?!?