Confused?!?

Jocelyn💋 • Have a wonderful life that I have to thank the Lord for❤️ have a son and expecting baby girl🌹
My baby girl was born on the 4th which was a Friday. Today she would be 13 days old. But the app is saying that today, (Thursday) she is 14 days old. She will be 14 days old tomorrow not today! Why is it saying that?