Online apps for ovulation are useless. They are not at all accurate, and are just meant to be used for fun. In order to predict ovulation you have to take your basal temperature and track ovulation for at least 6 months.
In other words, you can't go by the app.
Due dates can be off up to two weeks either way, so there is a chance that this other man is the father. The only way to find out is a DNA test after the baby is born. Anything else is a guess, and not at all accurate when determining the conception date and paternity.
Good luck.
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