Why the dates move
A sign boundary is not a calendar convention. It is the instant the Sun reaches a particular ecliptic longitude, and the first of those is the March equinox, the moment the Sun crosses the celestial equator going north. Everything else is measured in thirty degree steps from there.
The Earth takes about 365.2422 days to go round the Sun, which is not a whole number. Our calendar handles the remainder by inserting a leap day every four years, so the equinox drifts about six hours later each year and then jumps back roughly eighteen hours when February gains a day. Follow any one boundary across a decade and you will watch it move over a range of about a day and a half.
That is the whole reason a fixed range cannot be right. The table above solves the boundary directly for the year you choose rather than quoting an average of them.
The signs are not equal in length
The zodiac divides the ecliptic into twelve equal arcs of thirty degrees each. The Sun does not cross them at an equal rate. Earth's orbit is an ellipse, and Kepler's second law says a planet sweeps out equal areas in equal times, so it moves fastest when closest to the Sun and slowest when furthest away.
Perihelion, the closest point, falls in the first week of January. The Sun is in Capricorn then, so Capricorn and the signs either side of it are the short ones, near 29.5 days. Aphelion falls in early July with the Sun in Cancer, so Cancer and Gemini run closer to 31.5 days. The difference between the shortest and the longest sign is about two days, and no astrology site that publishes a fixed date range can show you that because the range hides it.
What to do if you were born on a cusp
There is no cusp. A cusp is what you get when someone hands you a date range accurate to the day and your birthday falls on one of the edges. It describes a gap in the information you were given, not a condition of the sky.
At any moment the Sun sits at one specific longitude, which is inside one sign. Born two hours before the boundary you are wholly the earlier sign, born two hours after it you are wholly the later one. Nothing is shared or blended.
To settle it you need three things: your birth date, your birth time, and the time zone that was actually in force where you were born, which for many places in the twentieth century was not the zone in force there now. Feed those into the sun sign calculator and it will give you your solar degree rather than a guess.
Why the times here are given in UTC
Every time in the table is Coordinated Universal Time, the modern successor to Greenwich Mean Time. That is the honest way to publish an astronomical event, because the event happens at one instant everywhere and only the local clock reading differs.
If Aries begins at 03:00 UTC on 20 March, then in Los Angeles that is 20:00 on 19 March, and someone born there at 21:00 on the nineteenth is an Aries even though a date range would have called them a Pisces. This is the single most common way sun signs get reported wrong, and it is entirely avoidable.
The tropical zodiac and the constellations
The signs used here are tropical, meaning they are measured from the March equinox rather than from the stars. Because of precession, the slow wobble of Earth's axis over about 25,800 years, the equinox point has drifted roughly one whole sign away from where it sat when the system was written down. The constellation the Sun is physically in front of on your birthday is usually one sign earlier than your sun sign.
This is the basis of the recurring news story that everyone's sign is wrong. It is not wrong so much as measuring something else. Western astrology has been explicitly seasonal for two thousand years: Aries means the spring equinox, not the stars of the Ram. Sidereal astrology, used in the Indian tradition, measures from the stars instead and produces the earlier sign. Both are internally consistent and they answer different questions.
Questions people ask about this
Why does every website give different star sign dates?
Because they are all rounding a moving boundary to a fixed day, and they round it differently. One site sees the Sun entering Aries on 20 March in most years and prints the 20th, another averages in the years it falls on the 21st. Neither is lying, and neither can answer a cusp birthday.
Can my sun sign be different from what I was told as a child?
Yes, and it happens most often to people born within a day of a boundary. The magazine range said one thing, the actual ingress time for your birth year and time zone says another. Checking takes a minute and the answer does not change again.
Which sign is the shortest?
Whichever one the Sun crosses nearest perihelion in early January, which in the current era means Capricorn and its neighbours run about 29.5 days. Cancer, crossed near aphelion in July, is the longest at roughly 31.5. The exact figures for your chosen year are in the days column above.
Do the sign dates change for the southern hemisphere?
No. The ingress happens at one instant for the whole planet, so the UTC times are identical. What differs is the season: Aries begins with the autumn equinox in Sydney rather than the spring one, which is a real and much-discussed problem for the seasonal reasoning behind the signs, but it does not move the dates.
How far ahead can I look these dates up?
The positions come from a numerical model of the solar system rather than a stored table, so any year from the picker works and the accuracy holds well beyond the range anyone needs for a birth chart.
What is an ingress?
The moment a body crosses from one sign into the next. The Sun's ingresses are the ones that define the calendar dates here. Every other planet has them too, and those are what mark the longer changes of tone that transit work is built on.