Confused

My last period was 19 may I'm supposed to be 9 weeks 1 day now and app says so too. For my first check up I went to scan doctor stated 24 Feb baby will be born and I'm 5 weeks 1 day. Today I went again and did scanning doctor said 27 Feb 2020 baby will be born. And I'm 8 weeks and 4 days. Which one am I supposed to follow. Even the gyne keep changing the expected birth date and my pregnancy weeks. Any advise how am I suppose to calculate? Thanks in advance.