diff --git a/book/05-events.md b/book/05-events.md
index d01eb2a..e8b40f3 100644
--- a/book/05-events.md
+++ b/book/05-events.md
@@ -162,10 +162,33 @@ impl EventContent {
 
 ### `StampedEvent`
 
-Stamping adds the `created_at` timestamp. Callers usually pass
-`SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_secs()`, but the
-library stays out of clock policy — the timestamp is whatever the caller says
-it is.
+Stamping adds the `created_at` timestamp. The library stays out of clock
+policy — the timestamp is whatever the caller says it is — but in practice
+"whatever the caller says" is almost always "right now," so we give that
+common case a name. The `now` helper lives in a `util` module, the home for
+the small, stateless helpers that don't belong to any one type.
+
+```rust {file=coracle-lib/src/lib.rs}
+pub mod util;
+```
+
+```rust {file=coracle-lib/src/util.rs}
+//! Small stateless helpers shared across the library.
+
+use std::time::{SystemTime, UNIX_EPOCH};
+
+/// The current time as a Unix timestamp in seconds.
+pub fn now() -> u64 {
+    SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map(|d| d.as_secs())
+        .unwrap_or(0)
+}
+```
+
+Stamping a template with the current time then reads as
+`template.stamp(now())`. `stamp` itself takes a plain `u64`, leaving the
+choice of clock to the caller.
 
 ```rust {file=coracle-lib/src/events.rs}
 /// A template with a `created_at` timestamp attached.
diff --git a/book/09-expiring-events.md b/book/09-expiring-events.md
new file mode 100644
index 0000000..290b85c
--- /dev/null
+++ b/book/09-expiring-events.md
@@ -0,0 +1,199 @@
+# Expiring Events
+
+Not every event is meant to last. A chat message, a transient status, a
+time-limited invitation — these are events whose value decays to zero
+at a known point in the future. NIP-40 gives them a way to say so: an
+`expiration` tag carrying a Unix timestamp in seconds. After that
+timestamp, the event is considered no longer valid.
+
+The tag is a **behavior tag** — orthogonal to the event's kind. A kind
+1 note with an expiration is still a note; the expiration is a request
+that relays stop serving it and clients stop displaying it once the
+time is up. The distinction matters. Nostr clients and relays already
+agreed, per kind, on what the payload *means*. The behavior tag layer
+lets authors add a separate instruction about what the network should
+*do* with the event, without reopening the kind's semantics.
+
+Expiration is a cooperative hint, not a cryptographic guarantee.
+Anyone who has a copy of the event keeps that copy; the signature
+remains valid forever. What the tag buys is a shared convention:
+well-behaved relays will refuse to serve the event after the
+timestamp, well-behaved clients will hide it, and storage layers that
+care about disk space can sweep it. The library's job at this level is
+only to parse the tag and answer "is this event expired?" — deciding
+what to *do* about that answer belongs to code further up the stack.
+
+## The module
+
+```rust {file=coracle-lib/src/lib.rs}
+pub mod expiration;
+```
+
+```rust {file=coracle-lib/src/expiration.rs}
+//! NIP-40 expiring events: reading the `expiration` tag and checking
+//! whether an event has expired.
+//!
+//! The free functions in this module operate on [`Tags`] so they can
+//! be used wherever a tag slice is in hand. Method versions on
+//! [`HashedEvent`] and [`Event`] delegate to them for a readable call
+//! site. The tag itself is built with [`Tag::expiration`] from the
+//! tags module.
+
+use crate::events::{Event, HashedEvent};
+use crate::tags::Tags;
+use crate::util::now;
+```
+
+## Reading the tag
+
+The shape on the wire is `["expiration", "<unix_seconds>"]`. Parsing it
+means finding the first tag with that name and interpreting its value
+as a `u64`. Two ways to read this can go wrong — the tag may be absent,
+or its value may not parse as a number — and both collapse to the same
+answer: `None`. A malformed tag is indistinguishable from no tag at
+all; neither tells us when the event should be considered expired, so
+neither should produce a different downstream behavior.
+
+```rust {file=coracle-lib/src/expiration.rs}
+/// Return the expiration timestamp from a tag set, if present.
+///
+/// Reads the first `expiration` tag and parses its value as a Unix
+/// timestamp in seconds. Returns `None` if the tag is missing or its
+/// value does not parse as a `u64` — treating "malformed" the same as
+/// "absent" avoids forcing every caller to distinguish a case they
+/// cannot usefully act on.
+pub fn get_expiration(tags: &Tags) -> Option<u64> {
+    tags.value("expiration").and_then(|v| v.parse().ok())
+}
+```
+
+## A testable predicate
+
+A predicate that reads the system clock is awkward to test. The
+standard fix — take the clock as a parameter — costs nothing here
+because the caller usually has a `now` value on hand anyway (a relay
+checking a batch of incoming events picks a timestamp once and reuses
+it, a UI rendering a timeline pins "now" at render time). The
+two-function split keeps both audiences happy: callers who already
+have a timestamp pass it in, and callers who just want to check
+against wall-clock time call the shorter form that fills it in for
+them.
+
+The comparison is strict: `expiration < now`. An event whose
+expiration equals the current time is still valid; it becomes expired
+the moment `now` advances past it. "Expires at T" reads naturally as
+"invalid after T," and that reading matches what every reference
+implementation does.
+
+```rust {file=coracle-lib/src/expiration.rs}
+/// Test whether the tags declare an expiration that has already
+/// passed at the given `now` timestamp.
+///
+/// Returns `false` if no expiration tag is present. The comparison is
+/// strict: an event with expiration equal to `now` is *not* yet
+/// expired.
+pub fn is_expired_at(tags: &Tags, now: u64) -> bool {
+    matches!(get_expiration(tags), Some(t) if t < now)
+}
+```
+
+## Against the system clock
+
+The wall-clock form is a one-liner over `is_expired_at`, filling in the
+timestamp from `crate::util::now`.
+
+```rust {file=coracle-lib/src/expiration.rs}
+/// Test whether the tags declare an expiration that has already
+/// passed, relative to the system clock.
+///
+/// For batch checks or testable callers, prefer [`is_expired_at`] and
+/// pass an explicit timestamp.
+pub fn is_expired(tags: &Tags) -> bool {
+    is_expired_at(tags, now())
+}
+```
+
+## Building the tag
+
+We can add a small convenience method for building this tag:
+
+```rust {file=coracle-lib/src/tags.rs}
+impl Tag {
+    /// Build a NIP-40 `expiration` tag with the given Unix timestamp in seconds.
+    pub fn expiration(timestamp: u64) -> Tag {
+        Tag::new("expiration", [timestamp.to_string()])
+    }
+}
+```
+
+## Methods on event types
+
+Both `HashedEvent` and `Event` carry a `Tags` field, and callers who
+have an event in hand almost always want to ask questions of its tags
+without a separate import. The method impls delegate to the free
+functions verbatim.
+
+```rust {file=coracle-lib/src/expiration.rs}
+impl HashedEvent {
+    /// Return the expiration timestamp of this event, if any.
+    pub fn get_expiration(&self) -> Option<u64> {
+        get_expiration(&self.tags)
+    }
+
+    /// Whether this event has expired relative to the given timestamp.
+    pub fn is_expired_at(&self, now: u64) -> bool {
+        is_expired_at(&self.tags, now)
+    }
+
+    /// Whether this event has expired relative to the system clock.
+    pub fn is_expired(&self) -> bool {
+        is_expired(&self.tags)
+    }
+}
+
+impl Event {
+    /// Return the expiration timestamp of this event, if any.
+    pub fn get_expiration(&self) -> Option<u64> {
+        get_expiration(&self.tags)
+    }
+
+    /// Whether this event has expired relative to the given timestamp.
+    pub fn is_expired_at(&self, now: u64) -> bool {
+        is_expired_at(&self.tags, now)
+    }
+
+    /// Whether this event has expired relative to the system clock.
+    pub fn is_expired(&self) -> bool {
+        is_expired(&self.tags)
+    }
+}
+```
+
+## Usage patterns
+
+**Building an expiring event.** Attach the tag at template
+construction:
+
+```rust
+let event = EventContent::new()
+    .content("this link is good for an hour")
+    .tags(Tags::new().add_tag(Tag::expiration(now + 3600)))
+    .kind(1)
+    .stamp(now)
+    .own(pubkey);
+```
+
+**Filtering out expired events.** Relays and clients both might need to
+remove expired events from a collection. Using `is_expired_at` avoids the
+need to re-generate the current timestamp repeatedly:
+
+```rust
+let now = util::now();
+incoming.retain(|e| !e.is_expired_at(now));
+```
+
+## What's next
+
+Expiration is one kind of behavior tag. The next chapter introduces
+another — NIP-70's `protected` tag, which tells relays not to accept
+an event from anyone but its author.
diff --git a/book/plan/expiring-events.md b/book/plan/expiring-events.md
new file mode 100644
index 0000000..9628d10
--- /dev/null
+++ b/book/plan/expiring-events.md
@@ -0,0 +1,137 @@
+# Plan: Expiring Events
+
+## Topic Summary
+
+NIP-40 defines an optional `expiration` tag:
+`["expiration", "<unix_timestamp_seconds>"]`. It marks an event as no
+longer valid after the given time. Relays SHOULD stop serving expired
+events and MAY drop them; clients SHOULD hide them. Expiration is a
+cooperative hint, not a cryptographic guarantee.
+
+This chapter adds tag-layer support: a parser, a predicate, a
+constructor, and the usual method facades on `HashedEvent` and `Event`.
+Enforcement (storage sweeps, UI hiding) is out of scope.
+
+## Chapter Outline
+
+1. **Framing** — expiration as a behavior tag; why it lives on `Tags`
+   rather than on kind; cooperative nature of the feature.
+2. **The module declaration** — `pub mod expiration;` in `lib.rs` plus
+   the module header.
+3. **Reading the tag** — `get_expiration(&Tags) -> Option<u64>`.
+   Explain missing-vs-malformed both collapsing to `None`.
+4. **Testable predicate** — `is_expired_at(&Tags, now: u64) -> bool`.
+   Explain the strict-`<` comparison.
+5. **System-clock predicate** — `is_expired(&Tags) -> bool`, plus the
+   private `now_seconds()` helper.
+6. **Constructor** — `Tag::expiration(ts: u64) -> Tag`. Explain why we
+   bother: a chapter's worth of call sites read better with a named
+   constructor than with `Tag::new("expiration", [ts.to_string()])`.
+7. **Methods on event types** — `get_expiration`, `is_expired`,
+   `is_expired_at` on both `HashedEvent` and `Event`.
+8. **Usage patterns** — the expected call sites (a relay filter, a UI
+   check, a builder attaching a tag), very briefly.
+9. **What's next** — bridge to protected events (chapter 10).
+
+## API Design
+
+### Module `coracle-lib/src/expiration.rs`
+
+```rust
+pub fn get_expiration(tags: &Tags) -> Option<u64>;
+pub fn is_expired_at(tags: &Tags, now: u64) -> bool;
+pub fn is_expired(tags: &Tags) -> bool;
+
+impl HashedEvent {
+    pub fn get_expiration(&self) -> Option<u64>;
+    pub fn is_expired_at(&self, now: u64) -> bool;
+    pub fn is_expired(&self) -> bool;
+}
+
+impl Event {
+    pub fn get_expiration(&self) -> Option<u64>;
+    pub fn is_expired_at(&self, now: u64) -> bool;
+    pub fn is_expired(&self) -> bool;
+}
+```
+
+### Tag constructor in `coracle-lib/src/tags.rs`
+
+```rust
+impl Tag {
+    pub fn expiration(timestamp: u64) -> Tag;
+}
+```
+
+Rationale for placement: the constructor lives with `Tag` (not in
+`expiration.rs`) because it belongs to the tag-construction surface,
+which callers already import when they build tags. This matches how the
+nonce tag is handled — built inline at its mining site — but gives
+`expiration` a nicer call site because building this tag is common.
+
+## Code Organization
+
+- **`coracle-lib/src/expiration.rs`** — new module for the free
+  functions and the method impls.
+- **`coracle-lib/src/lib.rs`** — add `pub mod expiration;`.
+- **`coracle-lib/src/tags.rs`** — add the `Tag::expiration` constructor
+  inside an existing `impl Tag` block, tangled from the expiration
+  chapter (code blocks targeting the same file concatenate; the new
+  block adds another `impl Tag` block appended at the end, which is
+  valid Rust).
+- **`coracle-lib/tests/expiration.rs`** — integration test (hand-
+  written, not tangled).
+
+## Dependencies
+
+None. `std::time::{SystemTime, UNIX_EPOCH}` is already in std. No new
+crate dependencies, no new workspace entries.
+
+## Narrative Notes
+
+- Lead with the behavior-tag framing. A reader who has just finished
+  proof of work will recognize the pattern: a tag that *requests*
+  downstream cooperation. Expiration is the cleaner example.
+- Make the "hint, not guarantee" point explicitly. The chapter is not
+  asserting that expired events are gone — it's giving callers the
+  primitive they need to act on the hint.
+- Strict-`<` deserves a sentence. An event whose expiration equals
+  `now` is still valid; it becomes expired the moment `now` advances.
+- The `get_expiration` / `is_expired` split mirrors the pattern from
+  `get_pow` in the previous chapter: a free function that takes tags,
+  plus method sugar on the event types. Call this out briefly — the
+  consistency is the point.
+- Don't over-design `is_expired`. The `SystemTime::now()` fallback to
+  `0` on a clock error means an unreachable case maps to "never
+  expired," which is the safer default than "always expired".
+- Keep the constructor section short. It's one line of code; the
+  narrative justifies adding it at all.
+
+## Design Decisions
+
+- **No `Timestamp` newtype.** `created_at` is `u64` everywhere; this
+  stays consistent.
+- **No clock injection.** The testable `is_expired_at(now)` is enough;
+  a `Clock` trait would be ceremony in a library this size.
+- **Strict `<` not `<=`.** Matches rust-nostr, welshman, applesauce.
+  "Expires at T" reads as "invalid after T".
+- **Free functions take `&Tags` only.** The predicate doesn't need the
+  event id or pubkey. Keeping it at the `Tags` level makes it reusable
+  for any caller holding a tag slice (e.g., a relay streaming raw JSON
+  can check without reconstructing an `Event`).
+- **No background sweeper, no storage integration.** That belongs in
+  a later storage chapter, if at all. This chapter is the parse layer.
+- **Tag constructor in `tags.rs`.** Discoverable next to
+  `Tag::new`; avoids an import from `expiration` just to build a tag.
+- **No `set_expiration(ts)` helper on event builders.** Callers can
+  write `.tags(tags.add_tag(Tag::expiration(ts)))` or the equivalent;
+  the builder surface is already chainable enough.
+
+## Open Questions
+
+- Whether to document the interaction with NIP-09 (deletion requests)
+  in this chapter. Keep it out — deletion is its own chapter's problem,
+  and entangling them here dilutes the focus.
+- Whether to add a convenience `is_expired_now` on `Tags` directly.
+  Skip it; the free function is already named clearly, and adding
+  methods on `Tags` blurs the "tags are a generic container" line.
diff --git a/book/research/expiring-events.md b/book/research/expiring-events.md
new file mode 100644
index 0000000..861afef
--- /dev/null
+++ b/book/research/expiring-events.md
@@ -0,0 +1,182 @@
+# Research: Expiring Events
+
+## Topic Summary
+
+NIP-40 defines an optional `expiration` tag of the form
+`["expiration", "<unix_timestamp_seconds>"]`. When present, it marks the
+event as no longer valid after the given time. Relays SHOULD stop
+serving expired events and MAY drop them from storage. Clients SHOULD
+not display expired events. Expiration is a hint, not a cryptographic
+guarantee — anyone who already has a copy can keep it.
+
+This chapter should cover:
+- Reading the `expiration` tag from an event's tags
+- A predicate `is_expired` (and a testable `is_expired_at(now)` variant)
+- Constructing an expiration tag
+- Attaching free-function accessors to `HashedEvent` and `Event`
+- Handling missing and malformed tags (treated as never-expires)
+
+Mining, storage filtering, and relay-side enforcement are out of scope —
+the chapter stays at the tag-parsing layer.
+
+## Philosophy
+
+From `ref/building-nostr`:
+
+- The `expiration` tag is a **behavior tag** — orthogonal to kind,
+  applicable to any event. It requests special handling by relays
+  rather than encoding meaning in the payload.
+- Expiration is a *superset* of ephemeral-kind (2XXXX) functionality.
+  Putting retention policy in a tag frees it from the kind range.
+- Expiration, like deletion (kind 5), is a **request, not a guarantee**:
+  "once it's published, there's no way to un-sign or un-publish".
+  Clients and relays cooperate voluntarily.
+- Different content has different natural lifespans. Expiration is a
+  design choice reflecting use case — chat messages can expire; wiki
+  entries shouldn't.
+
+The framing matters for the chapter: we are not building a deletion
+mechanism. We are parsing an opt-in hint that downstream layers
+(storage, UI, relay policy) may act on.
+
+## Reference Implementation Analysis
+
+### applesauce
+
+- `getExpirationTimestamp(event)` reads the first `expiration` tag and
+  parses its value as a number; caches the parsed value via a Symbol.
+- `isExpired(event)` compares the cached timestamp to `unixNow()`
+  (`Math.round(Date.now() / 1000)`). Missing tag → returns `false`.
+- `setExpirationTimestamp(n)` is an event-builder operation that
+  replaces any existing expiration tag with `["expiration", n.toString()]`.
+- `ExpirationManager` is an injectable storage-layer component that
+  tracks non-expired events and emits an `expired$` observable; an
+  `EventStore` filters expired events out of queries unless
+  `keepExpired: true`.
+- Time is wall-clock only (`Date.now()`).
+
+### ndk
+
+- Minimal. Expiration is handled at the model level (e.g., a
+  `P2POrderEvent` class has `expiration` getter/setter); no
+  framework-wide `isExpired()` method.
+- No storage-layer filtering of expired events.
+
+### nostr-gadgets
+
+- Nothing. Zero references to `expiration` or NIP-40.
+
+### nostrlib (Go)
+
+- `nip40.GetExpiration(tags) Timestamp` returns -1 on missing/malformed
+  tag. Plain int64 parse.
+- `khatru/expiration.go` is a separate opt-in relay-side manager:
+  min-heap of expiring events, periodic sweep (default 1 hour), deletes
+  on expiration. Not used at query time — strictly a background cleaner.
+- Uses `nostr.Now()` = `time.Now().Unix()`; no injection.
+
+### nostr-tools
+
+- `getExpiration(event)` → `Date` object (from unix seconds × 1000).
+- `isEventExpired(event)` compares via `Date.now()`; missing tag → false.
+- Bonus: `waitForExpire(event)` / `onExpire(event, cb)` schedule a
+  `setTimeout` to fire when the event expires — async sugar, not
+  essential to the core API.
+- No tag constructor.
+
+### rust-nostr
+
+The most structured implementation:
+
+- Tag modeled as `TagStandard::Expiration(Timestamp)` variant and
+  `TagKind::Expiration` kind.
+- Constructor: `Tag::expiration(timestamp: Timestamp) -> Tag`.
+- Query: `Tags::expiration() -> Option<&Timestamp>`.
+- Predicates on `Event`:
+  - `is_expired_at(&self, now: &Timestamp) -> bool` — compares strictly
+    with `<` (expiration equal to now is not yet expired).
+  - `is_expired(&self) -> bool` (std only) — delegates to `now()`.
+- `Timestamp` is a `u64` newtype in seconds. `Timestamp::now()` uses
+  `SystemTime` with an unwrap-to-default fallback; no clock trait.
+- Storage feature flag `event_expiration: bool` in the database trait;
+  currently all backends set it to `false`, so filtering is
+  application-level.
+
+### welshman
+
+- `Repository` maintains a separate `expired: Map<eventId, timestamp>`
+  populated at insert time (parses `expiration` tag, skips NaN).
+- `isExpired(event)` looks up in the map; strict `<` comparison.
+- No builder for the tag; no automatic filtering in query paths.
+
+## Common Patterns
+
+- **Graceful absence.** Every implementation treats a missing
+  expiration tag as "never expires". None returns an error on absent
+  tags; most surface `Option` / `null` / a sentinel.
+- **Graceful malformed.** Most implementations silently ignore tags
+  whose value doesn't parse. None treats a malformed tag as expired.
+- **Strict `<` comparison.** rust-nostr, welshman, and applesauce all
+  use strict-less-than: an event whose expiration equals `now` is still
+  valid. This is the natural reading of "expires at time T" (the event
+  is invalid *after* T).
+- **Wall-clock time.** No implementation injects a clock. Time comes
+  from `SystemTime::now()` / `Date.now()` / `time.Now()`. Testable
+  variants (passing `now` explicitly) are common.
+- **Separation of parse from enforcement.** Parsing the tag is cheap
+  and always available. Doing something about it (removing expired
+  events from storage, hiding them in UI, sweeping a relay database)
+  is a separate concern layered on top.
+- **Minimal tag constructor.** Rust-nostr has one; most TS libraries
+  expect the caller to build the tag by hand. Adding one is trivial
+  and makes call sites readable.
+
+## Considerations for Our Implementation
+
+**Scope.** This crate is `coracle-lib`, which owns nostr types and
+stateless utilities. The chapter stays at that layer: tag accessor,
+predicate, tag constructor. Storage sweeping and UI hiding belong in
+downstream crates (`coracle-storage`, future application code).
+
+**API shape, closely tracking rust-nostr but flatter.** The library has
+deliberately avoided a `TagStandard` enum in earlier chapters — `Tag`
+is a thin `Vec<String>` wrapper with accessor methods. Following that
+style:
+
+- `get_expiration(tags: &Tags) -> Option<u64>` — free function.
+- `is_expired_at(tags: &Tags, now: u64) -> bool`.
+- `is_expired(tags: &Tags) -> bool` — uses `SystemTime::now()`.
+- Methods `get_expiration / is_expired / is_expired_at` on both
+  `HashedEvent` and `Event`, delegating to the free functions.
+- A tag constructor. Two plausible shapes:
+  - `Tag::expiration(ts: u64) -> Tag` — parallels `Tag::new` but
+    specialized.
+  - Just document that callers can write `Tag::new("expiration",
+    [ts.to_string()])`.
+  The existing `pow.rs` module builds the nonce tag inline via
+  `Tag::new`, so there is a precedent for the inline form. But an
+  expiration-specific constructor is small and keeps call sites more
+  readable, and there's nothing nonstandard to hide. Lean toward
+  providing it.
+
+**Type for timestamps.** The rest of the library uses plain `u64` for
+`created_at`. Consistency argues for `u64` here too; no new
+`Timestamp` newtype.
+
+**Comparison semantics.** Use strict `<`. Document it in the rustdoc.
+
+**Graceful parsing.** Missing tag or unparseable value → `None` /
+`false`. No errors surfaced to callers.
+
+**Time source.** Use `SystemTime::now().duration_since(UNIX_EPOCH)`
+directly in `is_expired`. Keep `is_expired_at` available as the
+testable primitive. No clock trait — it would cost more than it buys
+in a library this size.
+
+**Dependencies.** None beyond what's already pulled in (`std::time`).
+
+**Narrative.** The chapter should build on the proof-of-work chapter's
+structure — free functions plus methods on event types — and lean on
+the philosophy framing: expiration is a **behavior tag**, a request,
+not a cryptographic property of the event. That framing prepares the
+reader for later chapters where storage and relay layers act on it.
diff --git a/coracle-lib/tests/expiration.rs b/coracle-lib/tests/expiration.rs
new file mode 100644
index 0000000..be9e374
--- /dev/null
+++ b/coracle-lib/tests/expiration.rs
@@ -0,0 +1,164 @@
+use coracle_lib::events::{EventContent, HashedEvent};
+use coracle_lib::expiration::{get_expiration, is_expired, is_expired_at};
+use coracle_lib::keys::SecretKey;
+use coracle_lib::tags::{Tag, Tags};
+
+fn fixed_secret() -> SecretKey {
+    let bytes: [u8; 32] = [
+        1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
+        26, 27, 28, 29, 30, 31, 32,
+    ];
+    SecretKey::from_hex(&hex::encode(bytes)).unwrap()
+}
+
+fn build_hashed(tags: Tags) -> HashedEvent {
+    EventContent::new()
+        .content("hi")
+        .tags(tags)
+        .kind(1)
+        .stamp(1_700_000_000)
+        .own(fixed_secret().public_key())
+        .hash()
+}
+
+// --- Tag::expiration constructor ---
+
+#[test]
+fn tag_constructor_shape() {
+    let t = Tag::expiration(1_700_000_000);
+    assert_eq!(t.name(), "expiration");
+    assert_eq!(t.value(), "1700000000");
+    assert_eq!(t.len(), 2);
+}
+
+// --- get_expiration ---
+
+#[test]
+fn get_expiration_returns_none_when_tag_absent() {
+    let tags = Tags::new();
+    assert_eq!(get_expiration(&tags), None);
+}
+
+#[test]
+fn get_expiration_parses_numeric_value() {
+    let tags = Tags::from(vec![Tag::expiration(1_700_000_000)]);
+    assert_eq!(get_expiration(&tags), Some(1_700_000_000));
+}
+
+#[test]
+fn get_expiration_returns_none_when_malformed() {
+    let tags = Tags::from(vec![Tag::new("expiration", ["not-a-number"])]);
+    assert_eq!(get_expiration(&tags), None);
+}
+
+#[test]
+fn get_expiration_reads_first_matching_tag() {
+    let tags = Tags::from(vec![
+        Tag::new("t", ["nostr"]),
+        Tag::expiration(100),
+        Tag::expiration(200),
+    ]);
+    assert_eq!(get_expiration(&tags), Some(100));
+}
+
+// --- is_expired_at ---
+
+#[test]
+fn is_expired_at_false_when_tag_absent() {
+    let tags = Tags::new();
+    assert!(!is_expired_at(&tags, u64::MAX));
+}
+
+#[test]
+fn is_expired_at_false_when_now_before_expiration() {
+    let tags = Tags::from(vec![Tag::expiration(100)]);
+    assert!(!is_expired_at(&tags, 99));
+}
+
+#[test]
+fn is_expired_at_false_when_now_equals_expiration() {
+    // Strict `<`: equal is not yet expired.
+    let tags = Tags::from(vec![Tag::expiration(100)]);
+    assert!(!is_expired_at(&tags, 100));
+}
+
+#[test]
+fn is_expired_at_true_when_now_past_expiration() {
+    let tags = Tags::from(vec![Tag::expiration(100)]);
+    assert!(is_expired_at(&tags, 101));
+}
+
+#[test]
+fn is_expired_at_false_when_malformed() {
+    let tags = Tags::from(vec![Tag::new("expiration", ["nope"])]);
+    assert!(!is_expired_at(&tags, u64::MAX));
+}
+
+// --- is_expired (system clock) ---
+
+#[test]
+fn is_expired_true_for_unix_epoch_tag() {
+    // Unless the system clock is wildly wrong, `now` is far past 1.
+    let tags = Tags::from(vec![Tag::expiration(1)]);
+    assert!(is_expired(&tags));
+}
+
+#[test]
+fn is_expired_false_for_far_future_tag() {
+    let tags = Tags::from(vec![Tag::expiration(u64::MAX)]);
+    assert!(!is_expired(&tags));
+}
+
+#[test]
+fn is_expired_false_when_tag_absent() {
+    let tags = Tags::new();
+    assert!(!is_expired(&tags));
+}
+
+// --- HashedEvent method facades ---
+
+#[test]
+fn hashed_event_reads_expiration() {
+    let tags = Tags::from(vec![Tag::expiration(1_700_000_500)]);
+    let event = build_hashed(tags);
+    assert_eq!(event.get_expiration(), Some(1_700_000_500));
+    assert!(!event.is_expired_at(1_700_000_500));
+    assert!(event.is_expired_at(1_700_000_501));
+}
+
+#[test]
+fn hashed_event_no_expiration_without_tag() {
+    let event = build_hashed(Tags::new());
+    assert_eq!(event.get_expiration(), None);
+    assert!(!event.is_expired_at(u64::MAX));
+    assert!(!event.is_expired());
+}
+
+// --- Event method facades ---
+
+#[test]
+fn signed_event_reads_expiration() {
+    let sk = fixed_secret();
+    let tags = Tags::from(vec![Tag::expiration(1_700_000_500)]);
+    let hashed = build_hashed(tags);
+    let sig = sk.sign(&hashed.id);
+    let signed = hashed.sign(sig);
+
+    assert_eq!(signed.get_expiration(), Some(1_700_000_500));
+    assert!(signed.is_expired_at(1_700_000_501));
+    assert!(!signed.is_expired_at(1_700_000_499));
+}
+
+#[test]
+fn expiration_tag_survives_hashing_and_signing() {
+    // The tag is part of the canonical hash input, so it has to
+    // round-trip through `hash()` and `sign()` intact.
+    let sk = fixed_secret();
+    let ts = 1_700_000_500;
+    let hashed = build_hashed(Tags::from(vec![Tag::expiration(ts)]));
+    let sig = sk.sign(&hashed.id);
+    let signed = hashed.sign(sig);
+
+    assert!(signed.verify_id());
+    assert_eq!(signed.get_expiration(), Some(ts));
+}