Your babies age..

How do you all calculate your babies age? I have been going by every Tuesday since he was born on a Tuesday, but I'm concerned because he was born on the 24th of June and his shot is scheduled for next Wednesday (Aug. 20th). So is he considered 2 months on the 24th or on the Tuesday (the 8 week mark) as I have been calculating it this far??

He is 2 months on the 24th, but 8 weeks on Tuesday 19th (remember a month is actually a bit longer than 4 weeks) x