diff --git a/AGENTS.md b/AGENTS.md
index 51157976..80c55db5 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -81,7 +81,7 @@ code.
   L6 depends on egui/eframe/rfd/winit. **The UI crate is swappable.**
 - **The UI is thin**: panels read engine state and act through `app.run(id, params)` /
   `app.set_props(ids, props)`. Colours come from `theme::Tokens`.
-- **Time goes through `chrono`** (workspace dependency). Never hand-roll UTC/calendar conversion (days-to-date algorithms, leap-year math); a crate owns only its output format string.
+- **Time goes through `chrono`** (workspace dependency). Never hand-roll UTC/calendar conversion (days-to-date algorithms, leap-year math); a crate owns only its output format string. The one exception is range extension: past the years chrono can represent, shift the date by whole 400-year Gregorian cycles (146,097 days, after which the calendar repeats), let chrono compute the month, day and time, and add the cycles back to the year.
 - **Rust only** (no handwritten JS/TS). **Never break wasm** (`cargo xtask wasm`).
 
 ### Quality gates (before every commit)
diff --git a/CLAUDE.md b/CLAUDE.md
index 51157976..80c55db5 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -81,7 +81,7 @@ code.
   L6 depends on egui/eframe/rfd/winit. **The UI crate is swappable.**
 - **The UI is thin**: panels read engine state and act through `app.run(id, params)` /
   `app.set_props(ids, props)`. Colours come from `theme::Tokens`.
-- **Time goes through `chrono`** (workspace dependency). Never hand-roll UTC/calendar conversion (days-to-date algorithms, leap-year math); a crate owns only its output format string.
+- **Time goes through `chrono`** (workspace dependency). Never hand-roll UTC/calendar conversion (days-to-date algorithms, leap-year math); a crate owns only its output format string. The one exception is range extension: past the years chrono can represent, shift the date by whole 400-year Gregorian cycles (146,097 days, after which the calendar repeats), let chrono compute the month, day and time, and add the cycles back to the year.
 - **Rust only** (no handwritten JS/TS). **Never break wasm** (`cargo xtask wasm`).
 
 ### Quality gates (before every commit)
diff --git a/Cargo.toml b/Cargo.toml
index 7bc0b082..b6fb2704 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -45,7 +45,7 @@ serde = { version = "1", features = ["derive", "rc"] }
 serde_json = { version = "1", features = ["float_roundtrip"] }
 regex = "1.13"
 log = "0.4"
-# All date/time conversion goes through chrono (no hand-rolled calendar code; AGENTS.md).
+# All date/time conversion goes through chrono (no hand-rolled calendar code beyond the 400-year range extension; AGENTS.md).
 chrono = { version = "0.4.45", default-features = false, features = ["std"] }
 proptest = "1"
 imbl = { version = "7", features = ["serde"] }