diff --git a/AGENTS.md b/AGENTS.md
index ea36711..a6e7eee 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -412,6 +412,22 @@ Notes:
 
 ---
 
+## Date & Time Formatting Rules
+
+- All date and time rendering must use the shared formatter in `src/lib/date-format.ts`.
+- Never use `Date.prototype.toLocaleString()`, `toLocaleDateString()`, `toLocaleTimeString()`, or `Intl.DateTimeFormat()` directly inside React components.
+- UI date display standard:
+  - Date: `YYYY-MM-DD`
+  - DateTime: `YYYY-MM-DD HH:mm:ss`
+  - Time: `HH:mm:ss`
+- API responses must remain ISO-8601 UTC.
+- UI is responsible for formatting.
+- Timezone is fixed to `Asia/Bangkok`.
+- Calendar system must always use Gregorian (AD), never Buddhist Era (BE).
+- All formatting must be deterministic to ensure identical SSR and Client rendering and prevent React hydration mismatches.
+
+---
+
 ## Authentication Patterns
 
 ### Protected Routes
diff --git a/docs/standards/project-foundations.md b/docs/standards/project-foundations.md
index 0b45988..a9734da 100644
--- a/docs/standards/project-foundations.md
+++ b/docs/standards/project-foundations.md
@@ -377,3 +377,17 @@ If ADR coverage is incomplete, this registry marks the gap explicitly.
 - ALLA OS Foundation Registry = COMPLETE
 - ALLA OS Reuse Discovery = READY
 - ALLA OS Governance Quality = IMPROVED
+
+## Date & Time Formatting Rules
+
+- All date and time rendering must use the shared formatter in `src/lib/date-format.ts`.
+- Never use `Date.prototype.toLocaleString()`, `toLocaleDateString()`, `toLocaleTimeString()`, or `Intl.DateTimeFormat()` directly inside React components.
+- UI date display standard:
+  - Date: `YYYY-MM-DD`
+  - DateTime: `YYYY-MM-DD HH:mm:ss`
+  - Time: `HH:mm:ss`
+- API responses must remain ISO-8601 UTC.
+- UI is responsible for formatting.
+- Timezone is fixed to `Asia/Bangkok`.
+- Calendar system must always use Gregorian (AD), never Buddhist Era (BE).
+- All formatting must be deterministic to ensure identical SSR and Client rendering and prevent React hydration mismatches.