How do I figure out dates

Alana • Baby girl Sophia-Louise 24-10-19 🍼 ❤️ Baby #2 due 03-08-21

Right ladies so my last period started on the 10 of January and I tested this morning out of curiosity and got 3+ anyone knows how to work it out would be greatly appreciated.. I don’t track my ovulation so that’s probably why it’s harder😒