The calculator adds your average cycle length to the first day of your last period. That is the whole calculation, and its accuracy depends entirely on how regular you are.
Cycle length is counted from the first day of one period to the day before the next. A normal cycle runs 21 to 35 days, and there is no medical significance to 28 specifically, despite it being treated as standard.
The first half of your cycle varies; the second half is relatively fixed at around 14 days after ovulation.
So anything that delays ovulation delays your period by the same amount. Stress, illness, travel across time zones, a sudden increase in training, significant weight change and disrupted sleep all do this.
A period arriving five days late usually means ovulation happened five days late, not that anything is wrong.
Cycles varying by a few days month to month is entirely normal. Research using large app datasets found that most women experience variation of around five days between their shortest and longest cycles.
What is worth attention:
The first reliable sign of perimenopause is usually a change in cycle length rather than symptoms.
Cycles often shorten first, sometimes to 24 or 25 days, because the follicular phase compresses. Later they lengthen and become erratic, with skipped months.
A persistent difference of seven days or more between consecutive cycles suggests early perimenopause. Gaps of 60 days or more suggest late perimenopause.
At that stage prediction stops being useful, which is itself informative.
Calendar-based prediction should not be relied on to avoid pregnancy. Typical-use failure rates are high, precisely because ovulation timing shifts in ways a calendar cannot detect.
Fertility awareness methods that track temperature and cervical mucus alongside dates perform considerably better, but they require training and consistency.
Related reading: The menstrual cycle explained · Irregular periods in perimenopause · Ovulation calculator · The first signs of perimenopause
Educational only, not medical advice, and not a contraceptive method.