Quick answer
Days until the astronomical start of summer. Enter today's date and hemisphere and the days until summer returns days until, date (UTC) and weekday.
Related tools: Days Until Fall: Days until the astronomical start of fall. Days Until Winter: Days until the astronomical start of winter. Days Until Spring: Days until the astronomical start of spring.
How to use the Days Until Summer
- Pick today's date.
- Choose hemisphere: Northern and Southern.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Today's date | A calendar date | — |
| Hemisphere | Choose one: Northern; Southern | Starts at Northern |
Formula and method
Formula
astronomical seasons begin at the equinoxes and solstices (the Sun's ecliptic longitude is 0°, 90°, 180° and 270°)
IEEE-754 double precision; shown to at most 6 decimal places.
Computed with Meeus's equinox and solstice algorithm. The date shown is the UTC date of the moment the season begins; in time zones far from UTC it can fall on the day before or after. Meteorological seasons (starting on the 1st of March, June, September and December) are different.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Today's date = 2025-01-01; Hemisphere = Northern.
| Result | Value |
|---|---|
| Days until | 171 |
| Date (UTC) | 2025-06-21 |
| Weekday | Saturday |
Example 2
Inputs: Today's date = 2025-06-21; Hemisphere = Northern.
| Result | Value |
|---|---|
| Days until | 0 |
| Date (UTC) | 2025-06-21 |
| Weekday | Saturday |
Example 3
Inputs: Today's date = 2026-12-22; Hemisphere = Northern.
| Result | Value |
|---|---|
| Days until | 181 |
| Date (UTC) | 2027-06-21 |
| Weekday | Monday |
The first line is the headline result (days until). Below it the days Until Summer lists date (UTC) and weekday, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
The June solstice
Astronomical summer begins at the June solstice, when the Sun's ecliptic longitude is 90° (northern hemisphere), and it brings the longest day of the year in the north. Meteorological summer starts on 1 June. This page counts to the solstice.
Longest day in the north
The June solstice falls on 20 or 21 June in most years. It is the day with the most daylight in the northern hemisphere and the least in the southern, where it marks the start of winter. The number of days to it changes by one when a leap year intervenes.
Start of northern-hemisphere summer (UTC)
Every figure in this table comes from running this calculator, so it matches what the form returns.
| Counting from | Date (UTC) | Weekday |
|---|---|---|
| 2026-01-01 | 2026-06-21 | Sunday |
| 2027-01-01 | 2027-06-21 | Monday |
| 2028-01-01 | 2028-06-20 | Tuesday |
| 2029-01-01 | 2029-06-21 | Thursday |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Today's date: left empty shows "Enter today's date." beside that field and no result.
Choosing between the Days Until Summer and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Days Until Fall](/date-calendar/days-until-fall/) | today's date and hemisphere | Days until |
| [Days Until Winter](/date-calendar/days-until-winter/) | today's date and hemisphere | Days until |
| [Days Until Spring](/date-calendar/days-until-spring/) | today's date and hemisphere | Days until |
| [Days Until Calculator](/date-calendar/days-until-calculator/) | target date | Days until |
| [Days Until December](/date-calendar/days-until-december/) | Days Until December |
Background: dates and times
Date and time results depend on the conventions a tool uses: whether the end date counts, whether weekends and holidays are skipped, how months of different lengths are handled and whether time zones are applied. This page states its convention, and the worked examples show it.
Dates use the Gregorian calendar and calendar days unless the page says otherwise; check deadlines against the rule in your contract or regulation.
Assumptions and limitations
- Southern-hemisphere seasons are offset by half a year
- The event date is in UTC
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for today's date to confirm this.
Frequently asked questions
Is the summer solstice the hottest day of the year?
No. The solstice has the most sunlight, but oceans and land keep warming afterwards, so the warmest weather usually comes weeks later.
What does the Days Until Summer calculate?
Days until the astronomical start of summer. It returns Days until, Date (UTC) and Weekday.
What formula does the Days Until Summer use?
astronomical seasons begin at the equinoxes and solstices (the Sun's ecliptic longitude is 0°, 90°, 180° and 270°). Computed with Meeus's equinox and solstice algorithm.
What do I need to enter in the days Until Summer?
You enter Today's date and Hemisphere.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Southern-hemisphere seasons are offset by half a year.
What are the limits of this calculator?
Southern-hemisphere seasons are offset by half a year. The event date is in UTC.
What does the “Hemisphere” option change?
It sets how the days Until Summer is worked out. The choices are Northern and Southern.
What is an example of a days Until Summer result?
With today's date 2025-01-01 and hemisphere Northern, the days until is 171.
Where does the method come from?
The method follows: Meeus, Astronomical Algorithms, 2nd ed. (1998), chapter 27; validated against JPL DE421 positions via Skyfield for 2020–2035 (maximum difference 1.3 minutes).
Related calculators
- Days Until Fall — Days until the astronomical start of fall.
- Days Until Winter — Days until the astronomical start of winter.
- Days Until Spring — Days until the astronomical start of spring.
- Days Until Calculator — Days remaining until any date (generalizes “days until June 15”, “days until Christmas” and similar pages).
- Days Until December — Days until 1 December.
- Days Until Christmas — Days until Christmas.
Sources
- Meeus, Astronomical Algorithms, 2nd ed. (1998), chapter 27; validated against JPL DE421 positions via Skyfield for 2020–2035 (maximum difference 1.3 minutes)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.