How we calculate every date

Every countdown on Days Until Next is computed, not typed. No date is hand-entered into a page, which is why they never go stale: the moment an event passes, its page resolves forward to the next occurrence on its own. Here is exactly how each kind of date is worked out.

Fixed-date events

The simplest case: an event that lands on the same calendar date every year — Christmas (December 25), Juneteenth (June 19), Halloween (October 31). We resolve the next occurrence of that month-and-day; if this year's has passed, we roll to next year.

Weekday-of-month rules

Many holidays are defined by a weekday, not a number: Thanksgiving is thefourth Thursday of November, Memorial Day is thelast Monday of May, Presidents' Day is the third Monday of February. We compute the exact date from that rule for the relevant year. US Election Day uses a compound version of the same idea — the first Tuesdayafter the first Monday of November.

Easter and movable feasts

Easter is the first Sunday after the first ecclesiastical full moon on or after the March equinox. We calculate it with the standardAnonymous Gregorian algorithm (the "computus"), and derive the dates that hang off it — such as the days before and after Easter Sunday — by a simple offset from that result.

Equinoxes and solstices

The start of each astronomical season is a precise moment, not a fixed date, and it drifts by a day or so from year to year. We compute the instant of each equinox and solstice using the algorithms in Jean Meeus'sAstronomical Algorithms (Chapter 27), accurate to about a minute — far more than enough to pin the calendar day.

Moon phases

Full moons and new moons are computed directly from the Moon's orbital phase using Meeus's method (Chapter 49), accurate to within a few minutes. Because the average gap from one full moon to the next — the synodic month — is about 29.53 days, the date shifts a little each month rather than repeating on a fixed day. We work in Universal Time (see timezones, below).

Eclipses

Eclipses can't be produced by a simple annual rule — they depend on the precise geometry of the Sun, Earth, and Moon, and arrive irregularly. Rather than approximate them, we read the next date from a hand-verified table built from NASA's eclipse catalogue. If a table is ever exhausted, the page fails loudly during our build rather than showing a stale date — so a wrong date never reaches you.

Scheduled events

Some countdowns — the Summer Olympics, the FIFA World Cup — have dates set by their organisers years ahead. These come from a small, curated table of published schedules, kept up to date by hand.

Calendar mechanics

Days of the week resolve to the next matching weekday; "the weekend" resolves to the next Saturday. Leap day is the next 29 February — leap years are those divisible by 4, except century years, unless they are also divisible by 400. Friday the 13th scans forward for the next 13th that falls on a Friday. The US daylight-saving change is the second Sunday of March (spring forward) and the first Sunday of November (fall back).

Timezones and Universal Time

Two things happen in different timezones. First, the live countdownon each page ticks down to midnight in your own timezone — that is what "days until" means to a person. Second, astronomical instants (moon phases, eclipses, solstices) are single moments in Universal Time, so the calendar date can differ by a day depending on where you are. When a printed calendar and one of these pages disagree by a day, that is almost always why, and we flag it on the pages where it matters.

How the pages stay correct

The same calculation runs in two places from one source of truth: when the page is built, to render the static answer you see with JavaScript disabled, and again in your browser, so the live ticker recomputes the correct next occurrence even if the page was built weeks ago. A "last updated" date reflects a genuine edit to a page's content — never simply the day it was last rebuilt.