Confused about third trimester beginning??

Hi all!

So I noticed on this app it says 26 weeks is the start of the third trimester. But most apps will say between 27-28 weeks...some even say 29 weeks (because I think some calculate the possibly of being pregnant for 42 weeks). I saw on some site, it depends on how you want to calculate it...based one conception, gestational, or development age will determine how you want to classify the start of your third trimester. I know many pregnant women will consider anywhere between 12-14 weeks as the start of your second trimester. What do y’all think?