A sign with no landmark at either end
Four of the twelve signs open on an event you can look up in an almanac: Aries at the March equinox, Cancer at the June solstice, Libra in September and Capricorn in December. Taurus is not one of them. Its boundaries are the points thirty and sixty degrees along from the equinox, and nothing visible happens in the sky when the Sun reaches them.
That does not make them vague. The longitude is calculable to the second, which is what the table above does. It simply means the dates have never had a folk marker attached to them the way midsummer has, and that in turn is part of why published date ranges for Taurus vary more than you would expect between one reference book and the next.
Why 20 April is not always right
Almost every list prints 20 April to 20 May. Both ends of that move. The tropical year is about a quarter of a day longer than the calendar year, so each boundary slides later by around six hours annually and then snaps back when February takes a leap day. Across a four year cycle the entry into Taurus ranges over roughly the 19th and 20th of April, and the exit over the 20th and 21st of May.
For most people this is trivia. For anyone born within a day of either edge it decides their sun sign, and the difference between an accurate answer and a wrong one is a single lookup in the table above.
Venus, earth, and the length of the month
Taurus is a fixed earth sign ruled by Venus, which is the reason it carries a reputation for slowness, comfort and stubbornness. Fixed signs hold the middle of a season rather than starting or ending it, earth signs are the ones that check the ground before moving, and Venus adds an interest in the physical pleasantness of the outcome. In the northern hemisphere the month lines up with the part of spring when the growth is already underway and nothing needs urgent decisions, which is where the association came from originally.
The season lasts a little over thirty and a half days, longer than the winter signs and shorter than the summer ones. Earth is on its way from perihelion in January towards aphelion in July, and it slows down steadily as it goes, so each sign from Capricorn onwards is fractionally longer than the last until Cancer. Taurus is one step up that ladder from Aries.
Borderline birthdays, and the view from the south
There is no Aries-Taurus cusp and no Taurus-Gemini cusp. A cusp is a description of missing information, not a state the Sun can be in. At any instant the Sun has one longitude and that longitude falls inside exactly one sign, so a birth thirty minutes before the crossing is fully Aries and a birth thirty minutes after it is fully Taurus. To resolve it you need a birth time and the historically correct time zone, including whatever summer time rule applied that year in that country.
South of the equator the dates are unchanged, since a solar ingress is one moment shared by the entire planet. The season is inverted, so Taurus falls in mid-autumn rather than mid-spring, and the harvest imagery attached to the sign belongs to the other hemisphere entirely.
Questions people ask about this
What are the Taurus dates?
Approximately 20 April to 20 May, but both ends shift by up to a day depending on the year. The table above gives the actual crossing times in UTC for whichever year you select, which is the only version that helps with a borderline birthday.
Why do some sites say Taurus starts on 21 April?
Older references often quote dates from earlier in the twentieth century, when the leap year drift placed the boundary later than it falls now. The ingress has crept slightly earlier over the last hundred years, so the printed ranges lag behind.
Is Taurus a spring sign?
In the northern hemisphere it is the middle month of spring, which is where its traditional imagery comes from. In the southern hemisphere the same dates fall in autumn. The dates themselves are global and do not change.
How long is Taurus season?
Around thirty and a half days, give or take a few hours. The signs cover equal arcs of sky but the Earth's orbital speed varies, so the time taken to cross each one differs by up to about two days between the shortest and the longest.
My birthday is 20 May, am I Taurus or Gemini?
It depends on your birth time and your birth location, and it genuinely goes either way in different years. Check the exit time in the table for your year of birth, convert it to the local zone in force at the time, and compare.