28 days is just one normal length among many: only about one cycle in eight runs exactly that long

What makes cycles differ is mostly the first half, before ovulation. The follicular phase averaged 16.9 days and varied far more than the second, luteal half, which held at around 12.4 days. Older women tended to have shorter cycles — about 0.18 days shorter per year of age between 25 and 45 — and the difference lay in the first half; the luteal phase barely differed across ages.

So the day of ovulation differs from woman to woman too — and you don't need app data to see it. Back in 2000, a BMJ study of 221 women planning a pregnancy found that the fertile window fell entirely between days 10 and 17 — where the classic rule places it — in only about 30% of them. For most it came earlier, for others much later, and even in usually regular cycles its timing could be unpredictable.

Your cycle owes nothing to a textbook number — it has its own rhythm, and that rhythm can shift over the years. Mark the first day of each period, and within a few months you'll see your own usual range rather than everyone's average; a calendar alone, though, can't tell you the day you ovulate. And if your cycle changes noticeably, that's a reason to talk to a doctor, not a reason to panic.