Settled at noon, but not by a wide margin
The Sun enters Taurus at thirty degrees of ecliptic longitude, an event that falls on 19 or 20 April in the years covered here. By noon on the twenty first the crossing has already happened in every one of those years, so the noon answer is Taurus each time.
That is not the same as saying the boundary is far away. A tropical year runs about a quarter day longer than the calendar allows for, so each ingress lands roughly six hours later than the year before, then springs back about eighteen hours at a leap day. Trace the Taurus ingress through one decade and it covers something like a day and a half of calendar. It has stayed on the near side of this date throughout the period, but the margin in the latest years of a leap cycle is measured in hours rather than days.
Where a birthplace could still overturn it
An ingress belongs to the whole planet at once, and only the local clock differs. When the Sun reaches thirty degrees at 22:00 UTC on 19 April, that instant is already the twentieth in Tokyo and still mid afternoon on the nineteenth in Vancouver. Anyone whose birth was recorded in a far eastern or far western time zone is reading a different calendar day for the same sky.
That is the mechanism behind most wrongly reported sun signs. The answer depends on a moment, and a moment needs a date, a time and a place. The widget above supplies all three, computes the ingress for your particular year, and reports how many hours you were born from it. On this date the figure is usually comfortable, but seeing the number is what makes it a fact rather than an assumption.
Cusp is a word for not knowing
Being told you are an Aries Taurus cusp sounds like a description of temperament. It is not one. The Sun sits at exactly one longitude in any given second, and that figure lies below thirty degrees or above it. There is no reading in which both are true, and no interval of blending on either side of the crossing.
What the term does describe is the state of the information. A writer with your calendar date but not your year, hour or city cannot pick a side, so a mixed description covers the risk. Add those details and the mixture has nothing left to explain.
Aries and Taurus compared fairly
Aries is cardinal fire ruled by Mars, read as impatient, direct and quick to begin. Taurus is fixed earth ruled by Venus, read as steady, sensory and reluctant to be hurried. They share no element and no modality, and cardinal giving way to fixed is the point in the sequence where starting turns into holding. On paper these are two of the less similar neighbours in the zodiac.
If the calculated answer surprises you, it moves one line of a chart. The Moon, the Ascendant, and every planetary position remain identical whichever side of the boundary the Sun fell on. Two people born a few hours apart across this line have charts that agree on nearly everything except the label most people quote.
Questions people ask about this
Is 21 April Aries or Taurus?
Taurus. In every year from 1930 to 2035 the Sun has already crossed into Taurus by noon on this date. The widget above confirms it against your own hour and birthplace.
Why do some ranges list 21 April as Aries?
Because a fixed printed range has to choose one cut off and cannot follow a boundary that moves. Older ranges in particular set the end of Aries a day later than the sky now supports.
Could an unusual year or time zone make it Aries?
It would take an extreme case. The margin is smallest in the years just before a leap day, and a birth in a far western time zone reads the calendar day differently, so the check is worth running rather than assuming.
Am I on the Aries Taurus cusp?
No such position exists. Your Sun occupied one longitude at birth and that longitude fell inside one sign. The word cusp signals that someone lacked your year, hour or birthplace.
Do I need my exact birth time?
Not on this date in any ordinary year, since the boundary sits a day or more away. It is worth having for the rest of a chart, because the Ascendant and the houses depend on it entirely.
Does an early degree of Taurus count as a full Taurus?
Yes. The sign applies from its first minute. Degree position affects aspects and house placement, not whether the sign itself is in force.