The calendar it runs on

Saju does not use the calendar on your wall, and it does not use the lunar calendar either, despite what almost every English-language article says.

It uses a solar calendar divided by the sun's actual position — twenty-four fixed points through the year called solar terms. Twelve of those act as boundaries between Saju months.

The February problem

The Saju year does not begin on 1 January. It does not begin at Lunar New Year. It begins at Ipchun, the start of spring, which falls around 4 February.

This catches an enormous number of people. If you were born in January or the first days of February, your Saju year is the previous one. Someone born on 3 February 2024 belongs to the Water Rabbit year. Someone born two days later belongs to the Wood Dragon year. They are not neighbours in the system; they are in different years.

A calculator that gets this wrong does not get one character wrong. The year pillar shifts, which shifts what everything else is measured against.

Months turn on the sun, not the first

The same logic applies within the year. Your month pillar does not change on the first of the month. It changes when the sun reaches a specific position, which lands a few days either side of the fourth of each calendar month.

This is why "I am a July person" means nothing in Saju until you know whether you were born before or after that month's boundary.

Why the lunar calendar confusion exists

Korea does use a lunar calendar for holidays and for traditional birthdays, and older Koreans often know their lunar birth date. So the two systems sit next to each other in everyday life and get conflated constantly in English writing about Saju.

If someone asks for your lunar birthday to calculate your Saju, they are either using a conversion step you do not need, or they do not know what they are doing.