How Do you calculate baby age - by weeks/ days of the month ?

If my baby was born on 5/29, then on 7/29 he would be 2 months old right? So 8 weeks? But the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> says on 7/29 he's 8 weeks 6 days. Which is correct?