diff --git a/CHANGELOG.md b/CHANGELOG.md
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--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,34 @@
 # changelog
 
+## 0.4.0
+
+The protocol/service split. `network.bsky.jetstream.*` is a Bluesky service
+API, not atproto — the same line Bluesky drew splitting `@bsky/sdk` out of
+`@atproto/api` at the Jetstream v2 launch (2026-08-13). zat is now the
+protocol side of that line.
+
+- **BREAKING**: `zat.ArchiveBackfill` (released through 0.3.30) moved to
+  [zat.dev/jetstream](https://tangled.org/zat.dev/jetstream), the zig
+  jetstream service SDK, along with the streamplace example, the archive
+  smoke script, and the vendored decode-only libzstd (zat links no zstd at
+  all now). Archive consumers: depend on the `jetstream` package; it
+  delivers the same `zat.JetstreamEvent`s through the same handler.
+- `zat.JetstreamClient` (the v1 `/subscribe` wire) remains here, functional
+  and deprecated-in-home, until its consumers migrate; the subscribeEvents
+  v2 client lands in the jetstream repo.
+- **feat**: `zat.Datetime` — the atproto datetime profile as a first-class
+  syntax type next to Tid/Did/Nsid/Rkey: `parse` (offsets normalized to
+  UTC, arbitrary fractional digits truncated at microseconds, the spec's
+  year-zero floor enforced on the normalized instant) and `toString`
+  (canonical `YYYY-MM-DDTHH:MM:SS.ffffffZ`). The atproto interop datetime
+  fixtures now drive this parser directly. Civil math is Hinnant's
+  days_from_civil/civil_from_days — signed and exact over 0000..9999,
+  unlike std.time.epoch. Deliberate divergences from general RFC 3339
+  parsers (reject leap-second `:60`, reject `-00:00`, upper-case T/Z only)
+  are the atproto spec's, cross-checked against rust's jiff and chrono and
+  documented in the type. Replaces four hand-rolled formatters in stream
+  and the SDK example's 45-line parser.
+
 ## 0.3.30
 
 Every phase of an HTTP fetch is now bounded. The 2026-08-13 zlay authority