Where the fixed dates come from
A sign begins when the sun crosses a particular point of ecliptic longitude — Aries at 0°, Taurus at 30°, and so on around the circle. Those crossings happen at a specific hour, and the hour lands on a slightly different calendar date each year, because the tropical year is not a whole number of days. Leap years pull it back; the years between push it forward.
“March 21 to April 19” is an average of those crossings, frozen for convenience. For most birthdays it is right. For birthdays within a day of a boundary it is a coin toss, and the two signs on either side of a boundary are read almost nothing alike.
What this site does instead
The sun's longitude is computed for the date given and the sign follows from that, rather than being looked up in a table. If you were born on a boundary day, the birth time matters and the pages say so instead of guessing.
There is a side effect worth naming: the result can disagree with the chart on the back of a magazine. When it does, the disagreement is the point rather than a bug — one of the two is working from an average.
A cusp is not a blend
Being born “on the cusp” is often described as being a mix of two signs. In the way this is actually calculated, that is not what happens. The sun is in one sign or the other at the moment of birth; there is no in-between state. What is true is that a sign's character tends to read faintly when the sun has only just entered it, and that is a different claim from being half of each.
Star signs next to Korean saju
Both work from your birth, but they read different units. A star sign comes from the month, the Chinese zodiac animal from the year, and the saju day pillar from the day. None of them replaces the others, and reading all three of yours is not self-contradiction — they are looking at different resolutions of the same moment.
In Korea the star signs are treated as the lightest of the three: something you know about yourself and mention, rather than something consulted before a decision. The pages here are written in that register.
Finding yours
The sign pages are written in Korean. The month index is mostly numbers and sign names, so it works without much reading.