Why equinox dates disagree: 22 or 23 September?

Look up the September 2026 equinox and you find two dates: 22 September on American pages, 23 September on European ones. Which is right? Both. The equinox is one instant for the whole planet, and its date depends on the clock you read it on.

In the equinox and solstice dates tool, the City field sets that clock. Without a city, the answer is in UTC (Coordinated Universal Time, the reference clock of time zones): 23 September, 00:05. In New York it is 20:05 on the 22nd, so the UTC date would put a New Yorker a day late.

Which field decides the date the tool shows?

The City field decides it, under From a place (optional). With no city, the tool writes each instant in UTC only. With a city, it adds a Local time column on that city’s clock, and there the date can change.

Pick the city from the suggestions that appear as you type. Picking fills in its coordinates and its time zone; a name typed without picking gives the UTC answer alone.

How can I tell which clock a time is on?

Read the header of the column: it names the clock. Instant (UTC) holds the moment in UTC. Local time (America/New_York) holds the same moment on New York’s clock, under its name in the IANA time zone database, the public reference for time zones.

With a city, the table Daylight on the four days names the local day as well. Its row reads “September equinox — Tuesday 22 September” for New York, and “September equinox — Wednesday 23 September” for Paris.

Does the date split for other equinoxes and solstices?

Yes, whenever the instant falls in the first hours after midnight UTC: the Americas are then still on the previous day. For New York, the tool shows it twice in the next two years.

Why does another site say 00:06 when the tool says 00:05?

Because the tool writes the minute in which the instant falls, and drops the seconds. Its calendar file keeps them: 00:05:16 UTC for this equinox. A table that rounds to the nearest minute, or computes the instant a few seconds differently, can print the next minute.

For this equinox, the US Naval Observatory gives 00:05 and the AstroPixels table gives 00:06. Across the 40 equinoxes and solstices of 2026 to 2035, the tool matched the Naval Observatory’s minute 24 times and was one minute earlier 16 times.

Questions

Which date should I use, 22 or 23 September?

Use the date on the clock of the people who will read it. For something local, that is your city’s date; for readers in several countries, give the UTC instant and write “UTC” after it.

Why do some sites say autumn starts on 1 September?

They use the meteorological seasons, which start on the first of March, June, September and December to keep whole months together in statistics. The tool gives the astronomical instants, so the northern autumn starts on 22 or 23 September, depending on your clock.

Is the equinox on a different date in the southern hemisphere?

Only where the clock differs: the date follows the time zone, not the hemisphere. What changes is the name of the season, since the September equinox opens spring in the south.

Sources and further reading

• US Naval Observatory, Earth’s Seasons — equinox and solstice times for 1700 to 2100, used here to check the tool’s minutes.
• AstroPixels, Solstices and Equinoxes: 2001 to 2100 — the table that gives 00:06 for the September 2026 equinox.
• IANA Time Zone Database — where zone names such as America/New_York, and their daylight saving rules, are kept.