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First Day of Autumn Calculator

The exact instant autumn begins — September equinox in the north, March equinox in the south — and how many minutes of daylight your latitude loses each day at that moment, the fastest rate of the year.

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First Day of Autumn Calculator is free to use as often as you like, directly from this page. You will find it under Dates & time, with First Day of Spring Calculator and First Day of Summer Calculator for the neighbouring cases.

How to use it

  1. Open the tool — no signup or install needed.
  2. Enter your input or adjust the available options.
  3. Get your result instantly, then copy or download it.

Frequently asked questions

What does First Day of Autumn Calculator do?

The exact instant autumn begins — September equinox in the north, March equinox in the south — and how many minutes of daylight your latitude loses each day at that moment, the fastest rate of the year.

When would I actually use this?

Answering a scheduling question exactly: how many working days until a deadline, what date falls sixty days out, and what time that is for a colleague elsewhere.

What is the most common mistake?

Counting days by subtracting dates and forgetting whether the endpoints are included. Inclusive and exclusive counts differ by one, which is the difference between meeting a deadline and missing it.

How is First Day of Autumn Calculator different from First Day of Spring Calculator?

They sit next to each other but answer different questions: First Day of Spring Calculator is the one to open when you need it to the exact instant spring begins in either hemisphere — March equinox in the north, September equinox in the south — in UTC and in your own offset, with the astronomical and meteorological start side by side and the next ten years. Pick whichever matches what you're starting from — both are free.

Is there a tool for the next step?

First Day of Summer Calculator is the closest one after this: The exact instant summer begins — June solstice in the north, December solstice in the south — plus the latitude above which the Sun never sets and why summer is the longest astronomical season.

What else is worth having open alongside it?

First Day of Winter Calculator and Birth Day of the Week Calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from?

Calendar arithmetic follows the proleptic Gregorian calendar and the IANA time-zone database, so daylight-saving transitions and leap years are handled. Public holidays vary by country and region and are not assumed.

Further reading

All guides →
ExplainerDate Arithmetic Is Harder Than It Looks"One month later" has no single correct answer, and every date library has had to pick one. Month addition is neither associative nor invertible, a day is not always 24 hours, and age is not days divided by 365.25.ExplainerLeap Years: The Rule, the Exception, and the Exception to the ExceptionDivisible by four, except centuries, except centuries divisible by four hundred. The three-line rule exists because the tropical year is not 365.25 days, and the arithmetic behind it explains ten deleted days in 1582 and a calendar that repeats exactly every 400 years.How-toHow to Convert Time Zones: UTC Offsets, Day Rollover and DSTConverting between time zones is adding or subtracting the difference in UTC offsets, then handling any day rollover. Learn the method, a worked example, and the DST caveat.How-toWhat Day of the Week Was I Born? Find Out in SecondsFind the weekday of any birth date. Learn the quick calculator method and a short introduction to the Doomsday rule for working it out in your head.ExplainerReading and Writing Military Time0800 is not “eight o'clock” and not “08:00” — it is “zero eight hundred”, with a zone letter on the end. The four digits, the spoken form, midnight at 0000, the argument about 2400, and the reason 12-hour notation cannot settle noon.ExplainerWorking Out an AnniversaryCounting back and counting forward are two different sums, and they disagree by a year on the day itself. What this calculator does with 29 February, what other date libraries do, and where the traditional and modern gift lists actually come from.