---
title: "Day Master"
method: POST
path: "/bazi/day-master"
tags: ["BaZi (Four Pillars)"]
---

# Day Master

`POST /bazi/day-master`

Day stem element + yin/yang polarity + canonical archetype description. The "self" character in BaZi from which all other pillars are interpreted.

## Request body

- BaziFourPillarsInput
  - `date` string, required
  - `time` string
  - `timezoneOffset` number

## Response `200`

Successful calculation

- object
  - `ok` boolean
  - `data` object
    - `dayMaster` object, nullable
      - `stem` string, nullable
      - `element` string, nullable
      - `yin` boolean, nullable
      - `polarityName` string, nullable
    - `dayPillar` object, nullable
      - `stem` string, nullable
      - `branch` string, nullable
      - `pillar` string, nullable
      - `animal` string, nullable
      - `stemElement` string, nullable
      - `branchElement` string, nullable
      - `yin` boolean, nullable
    - `interpretation` object, nullable
      - `summary` string, nullable
      - `archetype` string, nullable
    - `notes` string[], nullable
    - `disclaimer` string, nullable

## Other responses

- `400` — Validation error
- `401` — Missing or invalid API key

## Changes

- **2026-08-24** `8ab5a768b521` — 9 info
  - added the optional property `data/dayPillar/animal` to the response with the `200` status
  - added the optional property `data/dayPillar/branch` to the response with the `200` status
  - added the optional property `data/dayPillar/branchElement` to the response with the `200` status
  - added the optional property `data/dayPillar/pillar` to the response with the `200` status
  - …5 more
- **2026-08-22** `5a17388652f3` — 1 breaking, 2 info
  - added the new required request property `date`
  - added the new optional request property `time`
  - added the new optional request property `timezoneOffset`

[Change history](https://skmtc.dev/astroway/apis/astroway-calculation-api/changes/bazi/day-master/post.md)

---

[API](https://skmtc.dev/astroway/apis/astroway-calculation-api.md) · [All operations](https://skmtc.dev/astroway/apis/astroway-calculation-api/llms.txt) · [OpenAPI document](https://skmtc-service-production.skmtc.workers.dev/v1/apis/astroway/astroway-calculation-api/revisions/8ab5a768b521/schema)