Why one date gives two answers
Gemini begins at sixty degrees of ecliptic longitude. The Sun reaches that point some time on 20 or 21 May, and in the years where it arrives during the afternoon or evening of the twenty first, a birth earlier that day is Taurus.
The reason the moment will not stay still is that the tropical year is about a quarter of a day longer than the calendar year. Each successive year the ingress falls roughly six hours later by the clock, and then a leap day drags it back about eighteen hours. Traced across a decade the boundary sweeps over a day and a half of calendar time. That is why a mother and daughter born on the same date decades apart can be different signs, with neither of them mistaken.
Long signs drift more visibly
This boundary sits near the part of the year where the Earth is furthest from the Sun and moving most slowly along its orbit, so the Sun takes about thirty one and a half days to cross Gemini and Cancer, against about twenty nine and a half in the signs around early January. A longer sign means the entry point has more room to wander, which is one reason the late May and June boundaries feel less fixed than the winter ones.
It also means the answer depends on the hour more often than people expect. In a year when Gemini begins at eight in the evening UTC, someone born at midday on 21 May is Taurus and someone born at eleven that night is Gemini.
The place of birth is part of the question
An ingress is one instant everywhere at once, and only the local clock differs. When the Sun enters Gemini at 01:00 UTC on 21 May, that instant is nine in the evening on 20 May in New York and six in the evening in Los Angeles. So a Californian born on the evening of the twentieth is Gemini while a Londoner born at breakfast on the twenty first is also Gemini, and a Californian born at nine that morning is still Taurus.
Getting this conversion wrong is the ordinary way a sun sign ends up recorded incorrectly, and it is entirely fixable. The widget above takes the year, the clock time and the birthplace together, resolves them to a single moment, and reports how many hours from the boundary that moment sat.
Taurus, Gemini, and why there is no cusp between them
Taurus is fixed earth ruled by Venus, read as steady, sensory and reluctant to be hurried. Gemini is mutable air ruled by Mercury, read as quick, plural and easily redirected. On paper they share nothing, and this is a boundary where the popular contrast is broadly fair rather than overdrawn. Where they overlap is in a certain literalness: both are interested in what is actually in front of them, Taurus through the body and Gemini through language, and neither has much appetite for the mystical.
None of that comes in a mixture. At any instant the Sun holds one degree of longitude, which lies inside one sign, so a birth an hour before the change is entirely Taurus and one an hour after is entirely Gemini. When people describe themselves as a Taurus Gemini cusp they are usually describing an unanswered question about their birth details, not a state the Sun can be in. And whichever way it resolves, only the Sun is at stake. The Moon, the Ascendant and every other placement in the chart stand exactly where they stood.
Questions people ask about this
Is 21 May Taurus or Gemini?
Gemini in most years, Taurus in a substantial minority. Measured at noon between 1930 and 2035, seventy eight years were Gemini and twenty eight were Taurus. Your own year and hour decide which group you fall into.
Why do published date ranges differ for this boundary?
Because they compress a moving event into a fixed date. Some publishers end Taurus on 20 May and some on 21 May, and both are defensible summaries of a boundary that actually moves across both days.
Does my birth time change the answer?
On this date it can, and often does. When the ingress falls during 21 May itself, a morning birth and a late evening birth belong to different signs even in the same city.
Is a Taurus Gemini cusp a real placement?
No. The Sun sits at one ecliptic longitude at any given moment and that longitude belongs to a single sign. The cusp label describes information that has not been looked up rather than a blend of two signs.
Why is Gemini longer than some other signs?
Because the Earth is near its furthest point from the Sun in early July and moves more slowly there, so the Sun takes about thirty one and a half days to cross Gemini against about twenty nine and a half in the signs near early January.
I was born a few minutes after the change. Does that count?
It counts fully. A Sun at zero degrees of Gemini is as much in Gemini as one at fifteen degrees. Proximity to a boundary affects nothing about whether the sign applies.