I have two ovulation-calculator apps, it said. Different answers. One it said last week on 4/15 and the other one it said today 4/20. Confusing, help?

Dates. On a normal cycle your ovulation is 14 days before your next period. But since you can't foretell the future, this is of little help. You might have 11-17 days from the beginning of your period until ovulation. After ovulation it is always 14 days to your period. The apps have to assume a 25 day cycle or a 28 day or a 31 day cycle - thus the difference.