What actually gets marked
The milestones worth listing are not a fixed cultural set, so this page merges three kinds and sorts them into one timeline rather than making you keep three lists.
Month anniversaries run every month through the first two years, then every six months, because after a couple of years the monthly count stops being something anyone tracks. Year anniversaries run out to thirty. Round day counts — 100, 500, 1000, 10000 — are in because they are pleasantly arbitrary: they land on ordinary weekdays and are not on anyone's calendar, which makes them a better excuse than a date everybody already knew about.
Where two of these fall on the same day they are merged into one row rather than listed twice. That happens more often than you would guess: the 1000-day mark sits close to the two-and-three-quarter-year point, and near-collisions of that kind are exactly what makes a hand-kept list go wrong.
Day one, and why the count can differ by one
Some people count the first day as day one; others treat it as day zero and start counting from the next day. Neither is wrong, but two people using different conventions will disagree by exactly one day about when the hundredth day is, which is an argument with no correct answer available.
The box at the top decides it, and it is worth agreeing on before anyone books anything. Note that only the round day counts move — month and year anniversaries are calendar dates rather than counts, so the one-year date is the same day whichever way you count.
The end of the month problem
A relationship that starts on the 31st has no one-month date in a thirty-day month, and none at all in February. Something has to be chosen. This page clamps: the milestone falls on the last day of the shorter month, so January 31 gives February 28, or February 29 in a leap year.
The same rule handles a February 29 start date. Its year anniversaries fall on February 28 in common years and on the 29th in leap years. Plenty of people prefer March 1 for that case; there is no authority to appeal to, so pick one and be consistent. A February 29 birthday entered in the birthday fields is shown on the 28th in common years, with a label saying so.
Why there is a day-of-week column
A date is only half the information. A milestone falling on a Tuesday will in practice be marked on the Saturday before or after, and that is the day that needs to be free. Weekend dates are highlighted so you can see at a glance which ones fall conveniently and which need a plan.
The "book by" column is the milestone date minus the lead time you set, twenty-one days by default. That is a reasonable figure for a restaurant that takes reservations; a place people talk about wants longer, and anything involving travel wants longer still. If the book-by date has already gone, the column says so, which is the most useful thing this page does for an anniversary six weeks out.
Birthdays in the same list
The two optional birthday fields exist because these dates get planned together in practice. Each shows the next occurrence — this year's if it has not happened yet, next year's if it has — with the day of the week and the countdown. Seeing an anniversary and a birthday three weeks apart on the same screen is the point; that is a booking conflict you want to notice in advance rather than in the same week.
Related pages
For a countdown to a single date of any kind, with checkpoint dates and working-day counts, use the D-Day Calculator. Age from a date of birth, with its own set of arithmetic milestones, is the Age and Milestone Checker. Plain day counts between two dates are on the Date Calculator.
Questions people ask
Why do some rows list two milestones together?
Because they fall on the same calendar day. Rather than print the date twice, the labels are joined — a round day count and a month anniversary landing together read as one row.
What happens if I enter a start date in the future?
You get a countdown to it instead of a day count. Nothing else is calculated, since there is no elapsed time to measure yet.
Does the list stop at some point?
It covers month anniversaries for two years, year anniversaries to thirty, and round day counts to ten thousand — a little over twenty-seven years. Past all of that the table has nothing left to show and says so.
Can I use this for something other than a relationship?
Yes. It is a count of days since a chosen date with anniversary dates attached, so a job start date, a sobriety date or the day a business opened all work the same way.