Solar and lunar calendars
Which calendar your birthday is written in. Saju is calculated from the solar date.
In one sentence
Which calendar your birthday is written in. Saju is calculated from the solar date.
How it is used
Korean family records often keep a lunar birthday, so a reading converts it to the solar date first; get that step wrong and every pillar after it is wrong too. Leap months are the usual trap, and Korean and Chinese lunar tables disagree on a handful of dates, so a converted date is worth checking against what your family remembers. The lunar date itself never enters the calculation; it is only an input to convert.