Cycle is different between two apps

I use this app as well as Ovia. On this app it's telling me to expect my period Sunday the 27th- 29th but on Ovia it's telling me to expect the 29th-31st. The period length isn't weird, that's the normal length but all the data is the same in both apps. Does anyone else struggle with this? I'm hoping I don't get a period but I also don't know when is too early to test. I'm tired of wasting money on buying tests. 😡😤😰😨