If You're Born at 11 PM, Does the Saju Date Change?

Only half of it changes. Your Hour Pillar (時柱) shifts into the Zi hour starting at 11 PM, but your Day Pillar — the date of your Saju — stays anchored to midnight under Gwiraedang's convention, so it keeps your solar-calendar birthday as-is (the fixed-Zi method). That said, this stretch of time is genuine territory where conventions differ, and different perpetual calendars can land on a different Day Pillar for it.

What are the "night Zi hour" and "early Zi hour"?

The Zi hour spans the two hours from 11 PM to just before 1 AM the next day. Within that, the portion before midnight (11 PM-midnight) is called the night Zi hour (夜子時), and the portion after midnight (midnight-1 AM) is the early Zi hour (朝子時). Both calculate identically as the Zi hour for your Hour Pillar — what differs isn't the hour, but the date: whether someone born during the night Zi hour gets their Day Pillar read as that day, or the next.

My Day Pillar is a day off from another perpetual calendar

That can happen if you were born sometime around 11 PM. Some schools follow the night-Zi convention, rolling the Day Pillar over to the next day starting at 11 PM; others follow the fixed-Zi convention, keeping that day's Day Pillar all the way to midnight. Gwiraedang follows the fixed-Zi convention — anchored to your solar-calendar birthday — and states this disclosure directly in the correction details on your results screen. Neither convention is wrong; it's a difference in method. And for reference, the time correction for a daylight-saving birth is a separate layer of calculation from this convention question.

So which one is correct?

Honestly, there's no single settled answer here — it's genuine territory where schools disagree. What matters isn't deciding which convention is right, but knowing which convention the perpetual calendar you're using follows. Your Day Pillar is the central axis for reading yourself, so if there's a one-day discrepancy, it's worth considering both readings side by side. Either way, what comes out is, itself, not a fixed fate — just a mirror for reference.