A boundary that falls inside the date
Measured at noon from 1930 to 2035, the Sun on 30 April occupies from nine point two to ten point six degrees of Taurus. The decan boundary is exactly ten. Years at the low end of that band put the birth in the first decan, years at the high end put it in the second, and the difference is a matter of where the leap cycle had reached.
This is why a flat claim about the decan for this date should be treated with suspicion. Pages that assert one answer are usually working from a single reference year and presenting it as though it applied to everyone. The honest position is that 30 April can be either, and only a calculation for your own year and hour decides it.
The same caution applies to the printed decan date ranges that circulate widely. They are given as fixed spans, usually 20 to 30 April for the first Taurus decan, when in fact the edges move by up to a day and a half.
What changes if you land on the other side
The first decan of Taurus runs from zero to ten degrees. The Chaldean order of faces gives it to Mercury and the triplicity scheme gives it to Venus. The second decan runs from ten to twenty, and the same two schemes give it to the Moon and to Virgo respectively.
So crossing the line changes the traditional ruler under either method, from Mercury to the Moon in one scheme and from Venus to Virgo in the other. The readings that follow differ accordingly, the first band being described as the sign at its most characteristic and the second as more careful and more analytical. Whether that distinction carries any weight is an interpretive question, but the position it rests on is a measurable one.
Note also that decans are used by some schools and set aside entirely by others, so a reader who finds the whole layer unconvincing loses nothing from their sun sign by ignoring it.
The sign itself is not close to anything
None of this touches the sun sign. The Sun entered Taurus around ten days earlier and will not reach Gemini for another three weeks, so a 30 April birth is Taurus at noon in every year checked and at every hour of the day.
The steadiness comes from the leap year cycle keeping the calendar in step with the orbit. The Sun arrives about six hours later each ordinary year and is pulled back by a leap day, so the position on a fixed date wanders inside a band about a degree and a half wide and no further. That band happens to include a decan boundary on this particular date, which is unusual and worth knowing.
The parts a birthday cannot supply
Two placements matter at least as much as the Sun and neither is fixed by the date. The Moon changes sign every two and a half days or so, giving a 30 April birth two candidate signs at best. The rising sign cycles through all twelve in a single day, so it depends entirely on the hour and the place of birth.
A birthday tells you the least personal thing about a chart. The widget above goes further: it computes your solar degree from your own year and birthplace, which settles the decan question, and it reports how far the birth stood from the nearest sign boundary.
Questions people ask about this
Which Taurus decan is 30 April?
It depends on the year. The Sun runs from about nine to eleven degrees on this date and the boundary is at ten, so some years fall in the first decan and some in the second.
Is 30 April definitely Taurus?
Yes. The Sun is around ten degrees into the sign, more than a week past the Aries boundary and three weeks short of Gemini. No year or hour changes it.
What is the decan ruler if I fall in the first decan?
Mercury under the Chaldean order of faces and Venus under the triplicity scheme. If you fall in the second decan instead, the same two schemes give the Moon and Virgo.
How do I find out which side I am on?
Use the widget above with your birth year, time and place. It returns the exact solar longitude rather than a rounded figure, which is what the decan question turns on.
Does the birth hour matter here?
It can. The Sun advances about a degree a day, so within a year that already sits close to ten degrees, a morning birth and a late evening birth can differ by a few hundredths of a degree either side.
Does the decan boundary have the same status as a sign boundary?
Not quite. Sign boundaries are structural in every branch of the tradition, while decans are a subdivision used by some schools and ignored by others. The measurement is exact either way.