Where the next date comes from

What a cycle counts

MedlinePlus, the U.S. National Library of Medicine, puts it this way: "It is counted from the first day of one period to the first day of the next."

So the date this calculator asks for is the first day it started, not the day it ended. Mixing those two up shifts every result by several days.

Cycle length is the number of days between those two start days. If one started on 1 March and the next on 29 March, that cycle was 28 days.

The standard method

The Office on Women's Health writes: "The typical menstrual cycle is 28 days long, but each woman is different." That is why the default here is 28 days. The second half of that sentence matters just as much, which is why the field is there to be changed.

The same source writes: "Your periods are still regular if they usually come every 24 to 38 days." So when the length you enter falls outside 24 to 38 days, this calculator prints that one fact. It quotes a range from the source and passes no judgement.

The range we accept is slightly wider, 21 to 38 days, because someone with a shorter cycle than the source calls regular still wants to count days. Anything outside that we read as a typo and decline.

The next expected date is nothing more than those two values added together. The last start day plus one cycle, plus two, plus three. The screen prints that addition next to each date.

The detailed rules of this calculator

This section describes what the code does, in order. There are two inputs: the last start day and the average cycle length.

When you press calculate, we first count the days from that date to today. That count divided by the cycle length and rounded up gives the number of the first cycle we show. If you started 30 days ago with a 28 day cycle, the first cycle (day 28) has already gone by, so we start from the second. We list 4 of them.

If a cycle has already gone by, we print how many days have passed since that date, as a number and nothing else. We attach no meaning to it, because cycle length varies from month to month and one number tells you nothing on its own.

Adding days crosses month ends and leap days on its own. 31 January plus 28 days lands on 28 February, and a leap year rolls from 29 February to 1 March without drifting. A golden checks that crossing on every run.

If the date you enter is later than today, the calculation is declined and the reason is printed. The same happens for a date more than 400 days ago, or a cycle length outside 21 to 38 days.

Copying the result link only puts the date and cycle length into the address. Nothing is stored on a server, and opening the link runs the same calculation again in the reader's own browser.

Tips for entering

If you cannot recall the exact start day, an approximate date still works. A day out means the result is a day out, and no more.

If you do not know your average cycle length, leave it at 28, the figure the source calls typical. If you do know your own, entering it makes the result considerably closer.

If the gap varies by a few days each month, averaging the last few gaps is better than guessing. Turning the record on below works that average out for you.

When the record is on

The record is off by default, and turning it on asks you once more on its own. Article 23 of the Korean Personal Information Protection Act names information about health and sexual life as sensitive information. We never receive those values, but keeping them in this browser is still your call.

With it on, each start date you enter collects in this browser's storage and nowhere else. Once two or more have collected, we take the gaps between neighbouring dates, average them, and fill in the cycle length field. Gaps far outside a sensible range are read as typos and left out of the average.

Turning it off erases what was kept, right then. You can also erase without turning it off. On a shared computer it is better left off, because anything kept in the browser can be opened by the next person.

Where people get stuck

Entering the day it ended shifts every result by several days. The field asks for the first day it started.

If an expected date has gone by, this calculator tells you how many days, and stops there. It draws no conclusion from that number. If it is on your mind, ask a clinic rather than this screen.

If the result differs from last month, the cycle length or the start date you entered has most likely changed. The formula line under each date shows exactly which date had how many days added, so read that line first.

Opening the page on another device shows no records. Records stay in this browser and are never sent anywhere. That is the promise this app makes.