Does the City You Were Born In Change Your Saju?

It can. Even at the same clock time, a different longitude means a different position of the sun, and that can shift your Hour Pillar (時柱). Korea's standard time is set to 135°E, so Seoul's true solar time runs about 32 minutes behind the clock, and Busan about 24 minutes (roughly 4 minutes per degree of longitude, rounded). With this true-solar-time correction, a birth near an hour boundary can land in a different two-hour Hour Pillar slot. Gwiraedang applies this correction automatically once you select a birth city, and shows the details right on the results screen.

Are Seoul and Busan really different?

Both run behind the clock, and the gap between them is only about 8 minutes. So for most birth times, someone born in Seoul and someone born in Busan end up with the same Hour Pillar — the only cases that diverge are the narrow window where an hour boundary happens to fall between the two correction values. "A different city means a completely different chart" is an overstatement.

What is 135°E?

It's the reference meridian for Korea's standard time. Every country picks a reference longitude, and Korea uses 135°E, which runs near Akashi, Japan. The Korean peninsula sits west of that line, which is exactly why the clock and the sky's true time diverge. Korea briefly used 127.5°E before 1961, and Gwiraedang applies the standard 135°E consistently as most public perpetual calendars do — but discloses the fact if your birth falls in that earlier period. This is a separate layer of calculation from the daylight-saving correction.

I don't know my birth city, or I was born overseas

If you don't know your birth city, Gwiraedang calculates using Seoul as the default — the difference between cities within Korea is small enough that most results come out the same either way. If you were born abroad, simply selecting your city automatically handles that country's time zone, daylight saving, and longitude correction all at once — and if the corrected time lands around 11 PM, it may also overlap with the night Zi hour question. The city you were born in never made your Saju "wrong" — what matters is whether the calendar you're using knows about the correction, and the chart that comes out of it is, itself, not a fixed fate, just a mirror for reference.