diff --git a/Cargo.lock b/Cargo.lock
index a472ab512..ada579fc3 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4939,6 +4939,7 @@ name = "wordcraft-engine"
 version = "0.4.0"
 dependencies = [
  "image",
+ "js-sys",
  "log",
  "regex",
  "regex-automata",
diff --git a/crates/engine/Cargo.toml b/crates/engine/Cargo.toml
index f734417b7..cfa21ea44 100644
--- a/crates/engine/Cargo.toml
+++ b/crates/engine/Cargo.toml
@@ -27,5 +27,9 @@ regex = { workspace = true }
 regex-automata = { workspace = true }
 image = { workspace = true }
 
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+# The browser clock for `now_iso` (`SystemTime::now()` panics on wasm32).
+js-sys = "0.3"
+
 [dev-dependencies]
 zip = { workspace = true }
diff --git a/crates/engine/src/cmd/mod.rs b/crates/engine/src/cmd/mod.rs
index 596ff84d8..bce3564d9 100644
--- a/crates/engine/src/cmd/mod.rs
+++ b/crates/engine/src/cmd/mod.rs
@@ -212,19 +212,26 @@ pub fn split_para(s: &mut Session, at: &Pos) -> Result<Pos, CmdError> {
     Ok(new)
 }
 
+/// ISO-8601 timestamp (UTC, seconds) for `secs` since the Unix epoch.
+fn iso_from_unix_secs(secs: u64) -> String {
+    let (y, m, d) = civil_from_days((secs / 86_400) as i64);
+    let t = secs % 86_400;
+    format!("{y:04}-{m:02}-{d:02}T{:02}:{:02}:{:02}Z", t / 3600, t / 60 % 60, t % 60)
+}
+
 /// ISO-8601 timestamp (UTC, seconds).
 pub fn now_iso() -> String {
     #[cfg(not(target_arch = "wasm32"))]
     {
         let secs = std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH).map(|d| d.as_secs()).unwrap_or(0);
-        let days = (secs / 86_400) as i64;
-        let (y, m, d) = civil_from_days(days);
-        let t = secs % 86_400;
-        format!("{y:04}-{m:02}-{d:02}T{:02}:{:02}:{:02}Z", t / 3600, t / 60 % 60, t % 60)
+        iso_from_unix_secs(secs)
     }
+    // `SystemTime::now()` panics on wasm32-unknown-unknown, so ask the browser's clock.
     #[cfg(target_arch = "wasm32")]
     {
-        "2026-01-01T00:00:00Z".to_string()
+        let ms = js_sys::Date::now();
+        // Clamp a NaN or pre-epoch clock to 0 rather than fail.
+        iso_from_unix_secs(if ms.is_finite() && ms > 0.0 { (ms / 1000.0) as u64 } else { 0 })
     }
 }
 
@@ -246,3 +253,21 @@ pub fn civil_from_days(z: i64) -> (i64, u32, u32) {
 pub fn story_param(s: &Session, v: &Value) -> StoryRef {
     v.get("story").and_then(|x| serde_json::from_value(x.clone()).ok()).unwrap_or(s.sel.focus.story)
 }
+
+#[cfg(test)]
+mod iso_tests {
+    use super::iso_from_unix_secs;
+
+    #[test]
+    fn formats_epoch_and_known_timestamps() {
+        assert_eq!(iso_from_unix_secs(0), "1970-01-01T00:00:00Z");
+        assert_eq!(iso_from_unix_secs(1_760_082_785), "2025-10-10T07:53:05Z");
+    }
+
+    #[test]
+    fn formats_leap_day_and_year_end() {
+        assert_eq!(iso_from_unix_secs(1_709_164_800), "2024-02-29T00:00:00Z");
+        assert_eq!(iso_from_unix_secs(951_782_400), "2000-02-29T00:00:00Z");
+        assert_eq!(iso_from_unix_secs(1_735_689_599), "2024-12-31T23:59:59Z");
+    }
+}