The fertile window is about six days, ending on the day of ovulation — sperm survive several days, the egg roughly one.
Calendar prediction in an app is an estimate from past cycles. It works reasonably for regular cycles and poorly for irregular ones, which are exactly the cases where people most want it.
Basal body temperature rises after ovulation, so it confirms rather than predicts. Useful over months, useless for timing this month.
Ovulation predictor kits detect the LH surge and give roughly a day or two of warning. Cervical mucus changes are free and give similar notice with practice.
If cycles are regular and a year of trying has passed — six months over 35 — that is the point to seek help rather than a better tracking method.