How far past the boundary
At noon in every year from 1930 to 2035, the Sun on 23 May has stood between about one point four and two point eight degrees of Gemini. The Sun moves a little under one degree a day, so that puts the crossing roughly two days earlier in most years and never later than the twenty first.
Two degrees is a small number, but the relevant measure is time rather than degrees. Two degrees is about two days, and no birth hour or time zone shifts a chart by anything close to that. The sign is Gemini in every year on record.
The reason the crossing date wanders between the twentieth and the twenty first is the same leap year cycle that keeps it from wandering further. The moment the Sun reaches thirty degrees of ecliptic longitude falls about six hours later each year and then jumps back when February gains a day, so it circles within a day or so rather than sliding.
The first decan of Gemini
The first ten degrees of any sign make its first decan, and a 23 May Sun sits near the start of Gemini's. The Chaldean order, which deals the visible planets around the zodiac slowest first, reaches Jupiter for this band. The triplicity scheme takes a different route, assigning the three decans of Gemini to Gemini, Libra and Aquarius, which makes Mercury the ruler of the first.
So the two schemes disagree here, which is a useful thing to see. It is not a fault in either; they are different traditions with different logic, and the honest position is to name both rather than to present one as the answer. Under Jupiter the band is read as expansive and interested in scope; under Mercury it doubles the sign's own ruler and is read as the most characteristically Gemini part of Gemini.
The sign itself
Gemini is mutable air, ruled by Mercury. Mutable signs close a season and hand over to the next, and the tradition reads them as adaptable rather than as sustaining. Air groups Gemini with Libra and Aquarius, and is read as concerned with connection, language and the exchange of information.
Put together, the standard reading is quickness, curiosity and a certain restlessness. That is interpretation, applied identically to every Gemini birthday for a month, which is exactly why the degree and the decan are the parts of a date worth calculating.
Mercury rules the sign, which gives its own position a bearing on the Sun's affairs in the traditional reading. Mercury never travels far from the Sun, so for a late May birth it is usually in Taurus, Gemini or Cancer, but the year decides which and the date cannot. That is one more placement to calculate rather than assume.
What the date leaves open
The Moon travels about thirteen degrees a day and can change sign partway through 23 May, so two Moon signs stay possible until an hour is supplied. The rising sign changes roughly every two hours and could be any of the twelve.
If you have been told you are on the Taurus Gemini cusp because your birthday falls in late May, the widget above will give the actual distance. For this date it comes back as roughly two degrees of Gemini, which is to say two days past a boundary that a cusp label implies you are sitting on.
Questions people ask about this
Is 23 May Taurus or Gemini?
Gemini, in every year from 1930 to 2035 and at every hour. The Sun crosses out of Taurus around 20 or 21 May, so this date sits about two days inside the new sign.
Am I on the Taurus Gemini cusp?
No. The boundary is roughly two days behind you, and a cusp is not a real condition in any case. At any instant the Sun sits at one longitude, inside one sign.
What degree of Gemini is a 23 May Sun?
Between about one point four and two point eight, taken at noon across the years sampled. Your own position depends on your year, hour and birthplace.
Who rules the first decan of Gemini?
Jupiter in the Chaldean order and Mercury in the triplicity scheme, which routes the first decan of a sign back to the sign itself. Both are traditions rather than measurements, and naming both is the honest option.
Does an early degree mean a weaker Gemini?
No. A Sun at two degrees is as fully in Gemini as one at twenty. The degree matters for the decan and for aspects, not for whether the sign applies.
What does my birth time change?
The Moon sign, the rising sign and every house placement. It moves the Sun by a fraction of a degree and leaves the sign exactly where it is.