---
type: Palm Sutra Concept
title: "子時換日 (Zi-hour day boundary)"
description: "The Palm Sutra fixes the Day pillar from the lunar day."
resource: https://localpapa.com/tools/yizhang.html
category: "Damo Yi Zhang Jing · Schools"
tags: [yizhang, palm-sutra, school, convention, zi-hour, day-boundary]
timestamp: 2026-06-21T00:00:00Z
---

# 子時換日 (Zi-hour day boundary)

- **中文 / Chinese:** 子時換日

The Palm Sutra fixes the Day pillar from the lunar day. The Zi hour spans 23:00–01:00; the 23:00–24:00 "late Zi" is assigned by some schools to the current day and by others to the next, which shifts the Day pillar and the Hour pillar that follows it.

## Conventions 各派做法

- **Same day (common) (當日子時（通行）) — default** — Late Zi stays on the current day; the whole Zi hour uses the current day pillar.
- **Late Zi rolls over (夜子換日)** — Late Zi (23:00–24:00) rolls to the next day; the day pillar advances before the Zi hour is taken.

The [reading tool](https://localpapa.com/tools/yizhang.html) lets you pick the convention; the first option is the common default. Different choices can shift the pillars for births at late Zi or in a leap month.

---
*[All schools & conventions](./index.md) · [The four pillars](../pillars/index.md) · [Palm Sutra knowledge base](../index.md)*