Different Apps say different things

Kelsie
So I recently downloaded another period/ovulation track app just for fun and it is giving me a period start date of the 17th where as this one says the 14th - I put in all of the same info. Has anyone else seen this? It worries me that I messed up on my dates this month! 🙄 they make this getting pregnant thing sound SO EASY when you learn about it in school!!