First Day of Summer Calculator

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.

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.

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.

They sit next to each other but answer different questions: First Day of Autumn Calculator is the one to open when you need it to 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. Pick whichever matches what you're starting from — both are free.

First Day of Spring Calculator is the closest one after this: 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.

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

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.