diff --git a/CHANGELOG.md b/CHANGELOG.md
index 90dd48d1ac..32fc2b57c0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,35 @@
 
 Detailed changelog for Perry. See CLAUDE.md for concise summaries.
 
+## v0.5.998 — fix(date-fns): format() returns a formatted string instead of undefined
+
+**Symptom.** `import { format } from 'date-fns'; format(new Date(2020, 0, 6), 'yyyy-MM-dd')` compiled cleanly and ran without error, but the console printed `undefined` instead of `2020-01-06`. Same shape for every other free function date-fns exports (`parseISO`, `addDays`, `isAfter`, …).
+
+**Root cause — manifest entries existed, dispatch table did not.** Three layers were wired but the middle one was missing:
+
+1. `crates/perry-api-manifest/src/entries.rs` surfaces `method("date-fns", "format", false, None)` and friends, so the HIR-lowerer recognises the import and emits `NativeMethodCall { module: "date-fns", method: "format", … }`.
+2. `crates/perry-stdlib/src/dayjs.rs` defines `js_datefns_format` / `js_datefns_add_days` / etc and `runtime_decls.rs` declares them at the LLVM level.
+3. **Missing.** `NATIVE_MODULE_TABLE` in `crates/perry-codegen/src/lower_call.rs` had no `module: "date-fns"` rows, so `native_module_lookup("date-fns", false, "format", _)` returned `None` and the call fell through to the receiver-less-unknown-native branch, which materialises as undefined.
+
+Beyond the dispatch hole, `js_datefns_format` itself was a thin wrapper around `js_dayjs_format`, which only knows dayjs's *uppercase* tokens (`YYYY/DD`). date-fns uses *lowercase* Unicode-LDML tokens (`yyyy/dd`) — so even after the dispatch was hooked up, `format(d, 'yyyy-MM-dd')` would have printed the raw template back. And dayjs's formatter normalises to UTC, which shifts the displayed day across the midnight boundary for any non-UTC machine.
+
+**Fix.** Two halves:
+
+1. **Wire dispatch** (`crates/perry-codegen/src/lower_call.rs`). Added twelve `NativeModSig` rows for module `"date-fns"` covering the manifest surface (`format`, `parseISO`, `addDays/Months/Years`, `differenceInDays/Hours/Minutes`, `isAfter/Before`, `startOfDay/endOfDay`). All `has_receiver: false` — date-fns is a functional API, every call passes the date as the first argument.
+2. **Rewrite `js_datefns_format`** (`crates/perry-stdlib/src/dayjs.rs`). New `format_date_fns_pattern` helper walks the pattern character-by-character, recognises the LDML tokens date-fns actually emits (`y/M/d/H/h/m/s/S/a/E/X`), handles `'single-quoted literals'`, and formats in `chrono::Local` so the day-of-month matches Node's date-fns output on a non-UTC machine. Token-run length controls padding (e.g. `M` → `1`, `MM` → `01`, `MMM` → `Jan`, `MMMM` → `January`).
+
+**Validation.** New test fixture `test-files/test_date_fns_format.ts` exercises the regex-token path (existing) plus end-to-end `format(...)`. Byte-for-byte parity against Node's `--experimental-strip-types` on the original repro:
+
+```
+import { format } from 'date-fns';
+const d = new Date(2020, 0, 6);
+console.log(format(d, 'yyyy-MM-dd'));  // 2020-01-06
+console.log(format(d, 'yyyy'));         // 2020
+console.log(format(d, 'MM'));            // 01
+console.log(format(d, 'dd'));            // 06
+console.log(typeof format(d, 'yyyy'));   // string
+```
+
 ## v0.5.997 — feat(jsruntime): V8 named-import static-method dispatch for Effect.succeed
 
 **Symptom.** `import { Effect } from 'effect'; Effect.succeed(42)` returned the literal number `0` instead of an Effect class instance. `typeof e === 'number'`, `e.pipe` was `undefined`, `e._tag` was `undefined`. Same shape blocked any `import { X } from 'v8-fallback-pkg'; X.method(args)` pattern where the V8 module's top-level export `X` is itself a sub-namespace object holding the actual functions — the Effect/jose/many-internal-tools pattern.
diff --git a/CLAUDE.md b/CLAUDE.md
index 5a90de9706..24034a0ba2 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -8,7 +8,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 Perry is a native TypeScript compiler written in Rust that compiles TypeScript source code directly to native executables. It uses SWC for TypeScript parsing and LLVM for code generation.
 
-**Current Version:** 0.5.997
+**Current Version:** 0.5.998
 
 
 ## TypeScript Parity Status
diff --git a/Cargo.lock b/Cargo.lock
index e01bf2a6a9..a09635dc5b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4905,7 +4905,7 @@ checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
 
 [[package]]
 name = "perry"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "base64",
@@ -4960,14 +4960,14 @@ dependencies = [
 
 [[package]]
 name = "perry-api-manifest"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "serde",
 ]
 
 [[package]]
 name = "perry-codegen"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "log",
@@ -4980,7 +4980,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-arkts"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -4989,7 +4989,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-glance"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -4997,7 +4997,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-js"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "perry-dispatch",
@@ -5007,7 +5007,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-swiftui"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5016,7 +5016,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-wasm"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "base64",
@@ -5029,7 +5029,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-wear-tiles"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5037,7 +5037,7 @@ dependencies = [
 
 [[package]]
 name = "perry-diagnostics"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "serde",
  "serde_json",
@@ -5045,7 +5045,7 @@ dependencies = [
 
 [[package]]
 name = "perry-dispatch"
-version = "0.5.997"
+version = "0.5.998"
 
 [[package]]
 name = "perry-doc-fixture-my-bindings"
@@ -5056,7 +5056,7 @@ dependencies = [
 
 [[package]]
 name = "perry-doc-tests"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "clap",
@@ -5071,7 +5071,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-argon2"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "argon2",
  "perry-ffi",
@@ -5079,7 +5079,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-axios"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "reqwest",
@@ -5088,7 +5088,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-bcrypt"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "bcrypt",
  "perry-ffi",
@@ -5096,7 +5096,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-better-sqlite3"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "rusqlite",
@@ -5104,7 +5104,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-cheerio"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "scraper",
@@ -5112,14 +5112,14 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-commander"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-cron"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "chrono",
  "cron",
@@ -5128,7 +5128,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-dayjs"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5136,7 +5136,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-decimal"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "rust_decimal",
@@ -5144,7 +5144,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-dotenv"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "serde_json",
@@ -5152,7 +5152,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ethers"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "rand 0.8.6",
@@ -5160,21 +5160,21 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-events"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-exponential-backoff"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-fastify"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "bytes",
  "http-body-util",
@@ -5188,7 +5188,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-fetch"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "lazy_static",
  "perry-ffi",
@@ -5199,7 +5199,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-http"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "lazy_static",
  "perry-ext-http-server",
@@ -5211,7 +5211,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-http-server"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "bytes",
  "http-body-util",
@@ -5230,7 +5230,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ioredis"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "lazy_static",
  "perry-ffi",
@@ -5240,7 +5240,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-jsonwebtoken"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "base64",
  "jsonwebtoken",
@@ -5251,7 +5251,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-lru-cache"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "lru",
  "perry-ffi",
@@ -5259,7 +5259,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-moment"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5267,7 +5267,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-mongodb"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "bson",
  "futures-util",
@@ -5279,7 +5279,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-mysql2"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5289,7 +5289,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-nanoid"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "nanoid",
  "perry-ffi",
@@ -5298,7 +5298,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-net"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "rustls",
@@ -5309,7 +5309,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-nodemailer"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "lettre",
  "perry-ffi",
@@ -5319,7 +5319,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-pg"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "sqlx",
@@ -5328,7 +5328,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ratelimit"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "governor",
  "perry-ffi",
@@ -5336,7 +5336,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-sharp"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "base64",
  "image",
@@ -5345,14 +5345,14 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-slugify"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-streams"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "lazy_static",
  "perry-ffi",
@@ -5360,7 +5360,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-uuid"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "uuid",
@@ -5368,7 +5368,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-validator"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "perry-ffi",
  "regex",
@@ -5378,7 +5378,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ws"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "futures-util",
  "lazy_static",
@@ -5389,7 +5389,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-zlib"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "flate2",
  "perry-ffi",
@@ -5397,7 +5397,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ffi"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "dashmap 6.1.0",
  "once_cell",
@@ -5406,7 +5406,7 @@ dependencies = [
 
 [[package]]
 name = "perry-hir"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "perry-api-manifest",
@@ -5420,7 +5420,7 @@ dependencies = [
 
 [[package]]
 name = "perry-jsruntime"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "deno_core",
@@ -5440,7 +5440,7 @@ dependencies = [
 
 [[package]]
 name = "perry-parser"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "perry-diagnostics",
@@ -5452,7 +5452,7 @@ dependencies = [
 
 [[package]]
 name = "perry-runtime"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "base64",
@@ -5476,7 +5476,7 @@ dependencies = [
 
 [[package]]
 name = "perry-stdlib"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "aes 0.8.4",
  "aes-gcm",
@@ -5546,7 +5546,7 @@ dependencies = [
 
 [[package]]
 name = "perry-transform"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5556,7 +5556,7 @@ dependencies = [
 
 [[package]]
 name = "perry-types"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "anyhow",
  "thiserror 1.0.69",
@@ -5564,11 +5564,11 @@ dependencies = [
 
 [[package]]
 name = "perry-ui"
-version = "0.5.997"
+version = "0.5.998"
 
 [[package]]
 name = "perry-ui-android"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "itoa",
  "jni",
@@ -5583,7 +5583,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-geisterhand"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "rand 0.8.6",
  "serde",
@@ -5593,7 +5593,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-gtk4"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "cairo-rs",
  "dirs 5.0.1",
@@ -5612,7 +5612,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-ios"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "block2",
  "libc",
@@ -5627,7 +5627,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-macos"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "block2",
  "libc",
@@ -5645,11 +5645,11 @@ version = "0.1.0"
 
 [[package]]
 name = "perry-ui-testkit"
-version = "0.5.997"
+version = "0.5.998"
 
 [[package]]
 name = "perry-ui-tvos"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "block2",
  "libc",
@@ -5664,7 +5664,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-visionos"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "block2",
  "libc",
@@ -5679,7 +5679,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-watchos"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "block2",
  "libc",
@@ -5692,7 +5692,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-windows"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "libc",
  "perry-runtime",
@@ -5706,7 +5706,7 @@ dependencies = [
 
 [[package]]
 name = "perry-updater"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "base64",
  "ed25519-dalek",
@@ -5720,7 +5720,7 @@ dependencies = [
 
 [[package]]
 name = "perry-wasm-host"
-version = "0.5.997"
+version = "0.5.998"
 dependencies = [
  "wasmi",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index 263afa12d3..5bc9d23b70 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -190,7 +190,7 @@ opt-level = "s"       # Optimize for size in stdlib
 opt-level = 3
 
 [workspace.package]
-version = "0.5.997"
+version = "0.5.998"
 edition = "2021"
 license = "MIT"
 repository = "https://github.com/PerryTS/perry"
diff --git a/crates/perry-codegen/src/lower_call.rs b/crates/perry-codegen/src/lower_call.rs
index 35a7189303..ded2378c45 100644
--- a/crates/perry-codegen/src/lower_call.rs
+++ b/crates/perry-codegen/src/lower_call.rs
@@ -8975,6 +8975,121 @@ const NATIVE_MODULE_TABLE: &[NativeModSig] = &[
         args: &[],
         ret: NR_F64,
     },
+    // ========== date-fns ==========
+    // date-fns exports free functions: `format(date, pattern)`,
+    // `addDays(date, n)`, etc. The first argument is a Date (NaN-boxed
+    // f64 timestamp from `new Date(...)`). The manifest entries surface
+    // these as receiver-less NativeMethodCalls on module "date-fns".
+    // Without these rows the dispatch returns None and the call falls
+    // through to undefined. Refs date-fns format() blocker.
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "format",
+        class_filter: None,
+        runtime: "js_datefns_format",
+        args: &[NA_F64, NA_STR],
+        ret: NR_STR,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "parseISO",
+        class_filter: None,
+        runtime: "js_datefns_parse_iso",
+        args: &[NA_STR],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "addDays",
+        class_filter: None,
+        runtime: "js_datefns_add_days",
+        args: &[NA_F64, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "addMonths",
+        class_filter: None,
+        runtime: "js_datefns_add_months",
+        args: &[NA_F64, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "addYears",
+        class_filter: None,
+        runtime: "js_datefns_add_years",
+        args: &[NA_F64, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "differenceInDays",
+        class_filter: None,
+        runtime: "js_datefns_difference_in_days",
+        args: &[NA_F64, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "differenceInHours",
+        class_filter: None,
+        runtime: "js_datefns_difference_in_hours",
+        args: &[NA_F64, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "differenceInMinutes",
+        class_filter: None,
+        runtime: "js_datefns_difference_in_minutes",
+        args: &[NA_F64, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "isAfter",
+        class_filter: None,
+        runtime: "js_datefns_is_after",
+        args: &[NA_F64, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "isBefore",
+        class_filter: None,
+        runtime: "js_datefns_is_before",
+        args: &[NA_F64, NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "startOfDay",
+        class_filter: None,
+        runtime: "js_datefns_start_of_day",
+        args: &[NA_F64],
+        ret: NR_F64,
+    },
+    NativeModSig {
+        module: "date-fns",
+        has_receiver: false,
+        method: "endOfDay",
+        class_filter: None,
+        runtime: "js_datefns_end_of_day",
+        args: &[NA_F64],
+        ret: NR_F64,
+    },
     // ========== moment ==========
     // Only factory wired: moment instance methods take f64 handle (not i64),
     // incompatible with the has_receiver:true i64-first-arg dispatch ABI.
diff --git a/crates/perry-stdlib/src/dayjs.rs b/crates/perry-stdlib/src/dayjs.rs
index 83ade739b3..c2984fdcfb 100644
--- a/crates/perry-stdlib/src/dayjs.rs
+++ b/crates/perry-stdlib/src/dayjs.rs
@@ -520,16 +520,219 @@ pub unsafe extern "C" fn js_dayjs_is_valid(handle: Handle) -> f64 {
 // ============ date-fns compatible functions ============
 
 /// format(date, formatStr) -> string (date-fns compatible)
+///
+/// date-fns uses Unicode-LDML-ish tokens (lowercase `yyyy` for year,
+/// `dd` for day-of-month, `MM` for month, etc.) and formats in the
+/// **local** timezone — distinct from dayjs which uses uppercase
+/// `YYYY`/`DD` and is happy to format in UTC. We can't reuse
+/// `js_dayjs_format` for this because (a) the token replacements miss,
+/// and (b) local-vs-UTC swings the day across a midnight boundary.
+/// Refs date-fns blocker.
 #[no_mangle]
 pub unsafe extern "C" fn js_datefns_format(
     timestamp: f64,
     pattern_ptr: *const StringHeader,
 ) -> *mut StringHeader {
-    let handle_f64 = js_dayjs_from_timestamp(timestamp);
-    if handle_f64 == 0.0 {
+    use chrono::Local;
+    if timestamp.is_nan() {
         return std::ptr::null_mut();
     }
-    js_dayjs_format(f64_to_handle(handle_f64), pattern_ptr)
+    let pattern = string_from_header(pattern_ptr).unwrap_or_else(|| "yyyy-MM-dd".to_string());
+    // `new Date(year, monthIndex, ...)` stores a UTC ms timestamp whose
+    // wall-clock representation in local time is the literal components
+    // the user passed. So formatting it in `Local` reproduces those
+    // components — which is exactly what Node's date-fns does.
+    let secs = (timestamp / 1000.0) as i64;
+    let nanos = ((timestamp.rem_euclid(1000.0)) * 1_000_000.0) as u32;
+    let dt = match chrono::DateTime::from_timestamp(secs, nanos) {
+        Some(d) => d.with_timezone(&Local),
+        None => return std::ptr::null_mut(),
+    };
+    let formatted = format_date_fns_pattern(&pattern, &dt);
+    js_string_from_bytes(formatted.as_ptr(), formatted.len() as u32)
+}
+
+/// Translate a date-fns format pattern against a chrono datetime.
+///
+/// Handles the most common Unicode-LDML tokens that date-fns supports.
+/// Single-quoted literals (`'literal'`) pass through untouched. Tokens
+/// not recognized below pass through unchanged — matching date-fns'
+/// permissive behavior on unknown letters.
+fn format_date_fns_pattern<Tz: chrono::TimeZone>(pattern: &str, dt: &chrono::DateTime<Tz>) -> String
+where
+    Tz::Offset: std::fmt::Display,
+{
+    use chrono::{Datelike, Timelike};
+    let bytes = pattern.as_bytes();
+    let mut out = String::with_capacity(pattern.len() + 8);
+    let mut i = 0;
+    while i < bytes.len() {
+        let c = bytes[i];
+        // Single-quoted literal — anything between `'...'` is emitted
+        // verbatim. `''` is a single quote.
+        if c == b'\'' {
+            i += 1;
+            if i < bytes.len() && bytes[i] == b'\'' {
+                out.push('\'');
+                i += 1;
+                continue;
+            }
+            while i < bytes.len() && bytes[i] != b'\'' {
+                out.push(bytes[i] as char);
+                i += 1;
+            }
+            if i < bytes.len() {
+                i += 1;
+            }
+            continue;
+        }
+        if !c.is_ascii_alphabetic() {
+            out.push(c as char);
+            i += 1;
+            continue;
+        }
+        // Read a run of the same letter.
+        let start = i;
+        while i < bytes.len() && bytes[i] == c {
+            i += 1;
+        }
+        let run = i - start;
+        match c {
+            b'y' => match run {
+                1 => out.push_str(&format!("{}", dt.year())),
+                2 => out.push_str(&format!("{:02}", dt.year() % 100)),
+                _ => out.push_str(&format!("{:0width$}", dt.year(), width = run)),
+            },
+            b'M' => match run {
+                1 => out.push_str(&format!("{}", dt.month())),
+                2 => out.push_str(&format!("{:02}", dt.month())),
+                3 => out.push_str(short_month_name(dt.month())),
+                _ => out.push_str(long_month_name(dt.month())),
+            },
+            b'd' => match run {
+                1 => out.push_str(&format!("{}", dt.day())),
+                _ => out.push_str(&format!("{:02}", dt.day())),
+            },
+            b'H' => match run {
+                1 => out.push_str(&format!("{}", dt.hour())),
+                _ => out.push_str(&format!("{:02}", dt.hour())),
+            },
+            b'h' => {
+                let h12 = dt.hour() % 12;
+                let h12 = if h12 == 0 { 12 } else { h12 };
+                match run {
+                    1 => out.push_str(&format!("{}", h12)),
+                    _ => out.push_str(&format!("{:02}", h12)),
+                }
+            }
+            b'm' => match run {
+                1 => out.push_str(&format!("{}", dt.minute())),
+                _ => out.push_str(&format!("{:02}", dt.minute())),
+            },
+            b's' => match run {
+                1 => out.push_str(&format!("{}", dt.second())),
+                _ => out.push_str(&format!("{:02}", dt.second())),
+            },
+            b'S' => {
+                // Fractional seconds. date-fns uses S/SS/SSS for 1/2/3
+                // digits of millisecond precision.
+                let ms = dt.nanosecond() / 1_000_000;
+                match run {
+                    1 => out.push_str(&format!("{}", ms / 100)),
+                    2 => out.push_str(&format!("{:02}", ms / 10)),
+                    _ => out.push_str(&format!("{:03}", ms)),
+                }
+            }
+            b'a' => {
+                // am/pm marker
+                let ampm = if dt.hour() < 12 { "AM" } else { "PM" };
+                out.push_str(ampm);
+            }
+            b'E' => {
+                // Day-of-week abbreviations. EEEE = long, EEE/EE/E = short.
+                let wd = dt.weekday();
+                if run >= 4 {
+                    out.push_str(long_weekday_name(wd));
+                } else {
+                    out.push_str(short_weekday_name(wd));
+                }
+            }
+            b'X' | b'x' => {
+                // Time-zone offset. Approximate as ±HH:MM (date-fns has
+                // many variants; this covers the common case).
+                out.push_str(&format!("{}", dt.offset()));
+            }
+            _ => {
+                // Unknown letter run — emit verbatim (date-fns throws
+                // on truly unknown tokens, but conservatively passing
+                // through is closer to dayjs's behavior).
+                for _ in 0..run {
+                    out.push(c as char);
+                }
+            }
+        }
+    }
+    out
+}
+
+fn short_month_name(m: u32) -> &'static str {
+    match m {
+        1 => "Jan",
+        2 => "Feb",
+        3 => "Mar",
+        4 => "Apr",
+        5 => "May",
+        6 => "Jun",
+        7 => "Jul",
+        8 => "Aug",
+        9 => "Sep",
+        10 => "Oct",
+        11 => "Nov",
+        12 => "Dec",
+        _ => "",
+    }
+}
+
+fn long_month_name(m: u32) -> &'static str {
+    match m {
+        1 => "January",
+        2 => "February",
+        3 => "March",
+        4 => "April",
+        5 => "May",
+        6 => "June",
+        7 => "July",
+        8 => "August",
+        9 => "September",
+        10 => "October",
+        11 => "November",
+        12 => "December",
+        _ => "",
+    }
+}
+
+fn short_weekday_name(w: chrono::Weekday) -> &'static str {
+    match w {
+        chrono::Weekday::Mon => "Mon",
+        chrono::Weekday::Tue => "Tue",
+        chrono::Weekday::Wed => "Wed",
+        chrono::Weekday::Thu => "Thu",
+        chrono::Weekday::Fri => "Fri",
+        chrono::Weekday::Sat => "Sat",
+        chrono::Weekday::Sun => "Sun",
+    }
+}
+
+fn long_weekday_name(w: chrono::Weekday) -> &'static str {
+    match w {
+        chrono::Weekday::Mon => "Monday",
+        chrono::Weekday::Tue => "Tuesday",
+        chrono::Weekday::Wed => "Wednesday",
+        chrono::Weekday::Thu => "Thursday",
+        chrono::Weekday::Fri => "Friday",
+        chrono::Weekday::Sat => "Saturday",
+        chrono::Weekday::Sun => "Sunday",
+    }
 }
 
 /// parseISO(dateString) -> timestamp (date-fns compatible)
diff --git a/test-files/test_date_fns_format.ts b/test-files/test_date_fns_format.ts
index 6406d3e8ae..4b9abc4147 100644
--- a/test-files/test_date_fns_format.ts
+++ b/test-files/test_date_fns_format.ts
@@ -27,3 +27,13 @@ console.log("aabbbccdd".match(g));
 
 // No-match returns null
 console.log("xyz".match(/(\w)\1+/));
+
+// End-to-end: format(date, pattern). Uses the native runtime path
+// (date-fns is well-known-aliased to perry-ext-dayjs).
+import { format } from "date-fns";
+const d = new Date(2020, 0, 6);
+console.log(format(d, "yyyy-MM-dd"));
+console.log(format(d, "yyyy"));
+console.log(format(d, "MM"));
+console.log(format(d, "dd"));
+console.log(typeof format(d, "yyyy"));