feat: add local date/timezone to turn environment context (#12947)

## Summary

This PR includes the session's local date and timezone in the
model-visible environment context and persists that data in
`TurnContextItem`.

  ## What changed
- captures the current local date and IANA timezone when building a turn
context, with a UTC fallback if the timezone lookup fails
- includes current_date and timezone in the serialized
<environment_context> payload
- stores those fields on TurnContextItem so they survive rollout/history
handling, subagent review threads, and resume flows
- treats date/timezone changes as environment updates, so prompt caching
and context refresh logic do not silently reuse stale time context
- updates tests to validate the new environment fields without depending
on a single hardcoded environment-context string

## test

built a local build and saw it in the rollout file:
```
{"timestamp":"2026-02-26T21:39:50.737Z","type":"response_item","payload":{"type":"message","role":"user","content":[{"type":"input_text","text":"<environment_context>\n  <shell>zsh</shell>\n  <current_date>2026-02-26</current_date>\n  <timezone>America/Los_Angeles</timezone>\n</environment_context>"}]}}
```

codex-rs/core/src/codex.rs

@@ -53,6 +53,8 @@ use crate::util::error_or_panic;
 use crate::ws_version_from_features;
 use async_channel::Receiver;
 use async_channel::Sender;
+use chrono::Local;
+use chrono::Utc;
 use codex_hooks::HookEvent;
 use codex_hooks::HookEventAfterAgent;
 use codex_hooks::HookPayload;
@@ -595,6 +597,8 @@ pub(crate) struct TurnContext {
     /// the model as well as sandbox policies are resolved against this path
     /// instead of `std::env::current_dir()`.
     pub(crate) cwd: PathBuf,
+    pub(crate) current_date: Option<String>,
+    pub(crate) timezone: Option<String>,
     pub(crate) developer_instructions: Option<String>,
     pub(crate) compact_prompt: Option<String>,
     pub(crate) user_instructions: Option<String>,
@@ -679,6 +683,8 @@ impl TurnContext {
             reasoning_summary: self.reasoning_summary,
             session_source: self.session_source.clone(),
             cwd: self.cwd.clone(),
+            current_date: self.current_date.clone(),
+            timezone: self.timezone.clone(),
             developer_instructions: self.developer_instructions.clone(),
             compact_prompt: self.compact_prompt.clone(),
             user_instructions: self.user_instructions.clone(),
@@ -719,6 +725,8 @@ impl TurnContext {
         TurnContextItem {
             turn_id: Some(self.sub_id.clone()),
             cwd: self.cwd.clone(),
+            current_date: self.current_date.clone(),
+            timezone: self.timezone.clone(),
             approval_policy: self.approval_policy.value(),
             sandbox_policy: self.sandbox_policy.get().clone(),
             network: self.turn_context_network_item(),
@@ -748,6 +756,16 @@ impl TurnContext {
     }
 }
 
+fn local_time_context() -> (String, String) {
+    match iana_time_zone::get_timezone() {
+        Ok(timezone) => (Local::now().format("%Y-%m-%d").to_string(), timezone),
+        Err(_) => (
+            Utc::now().format("%Y-%m-%d").to_string(),
+            "Etc/UTC".to_string(),
+        ),
+    }
+}
+
 #[derive(Clone)]
 pub(crate) struct SessionConfiguration {
     /// Provider identifier ("openai", "openrouter", ...).
@@ -1017,6 +1035,7 @@ impl Session {
                 .features
                 .enabled(Feature::UseLinuxSandboxBwrap),
         ));
+        let (current_date, timezone) = local_time_context();
         TurnContext {
             sub_id,
             config: per_turn_config.clone(),
@@ -1028,6 +1047,8 @@ impl Session {
             reasoning_summary,
             session_source,
             cwd,
+            current_date: Some(current_date),
+            timezone: Some(timezone),
             developer_instructions: session_configuration.developer_instructions.clone(),
             compact_prompt: session_configuration.compact_prompt.clone(),
             user_instructions: session_configuration.user_instructions.clone(),
@@ -4676,6 +4697,8 @@ async fn spawn_review_thread(
         tools_config,
         features: parent_turn_context.features.clone(),
         ghost_snapshot: parent_turn_context.ghost_snapshot.clone(),
+        current_date: parent_turn_context.current_date.clone(),
+        timezone: parent_turn_context.timezone.clone(),
         developer_instructions: None,
         user_instructions: None,
         compact_prompt: parent_turn_context.compact_prompt.clone(),
@@ -7259,6 +7282,8 @@ mod tests {
         let previous_context_item = TurnContextItem {
             turn_id: Some(turn_context.sub_id.clone()),
             cwd: turn_context.cwd.clone(),
+            current_date: turn_context.current_date.clone(),
+            timezone: turn_context.timezone.clone(),
             approval_policy: turn_context.approval_policy.value(),
             sandbox_policy: turn_context.sandbox_policy.get().clone(),
             network: None,
@@ -7296,6 +7321,8 @@ mod tests {
         let mut previous_context_item = TurnContextItem {
             turn_id: Some(turn_context.sub_id.clone()),
             cwd: turn_context.cwd.clone(),
+            current_date: turn_context.current_date.clone(),
+            timezone: turn_context.timezone.clone(),
             approval_policy: turn_context.approval_policy.value(),
             sandbox_policy: turn_context.sandbox_policy.get().clone(),
             network: None,
@@ -7490,7 +7517,10 @@ mod tests {
             .record_context_updates_and_set_reference_context_item(&turn_context, None)
             .await;
         let history_after_second_seed = session.clone_history().await;
-        assert_eq!(expected, history_after_second_seed.raw_items());
+        assert_eq!(
+            history_after_seed.raw_items(),
+            history_after_second_seed.raw_items()
+        );
     }
 
     #[tokio::test]
@@ -7658,6 +7688,8 @@ mod tests {
         let previous_context_item = TurnContextItem {
             turn_id: Some(turn_context.sub_id.clone()),
             cwd: turn_context.cwd.clone(),
+            current_date: turn_context.current_date.clone(),
+            timezone: turn_context.timezone.clone(),
             approval_policy: turn_context.approval_policy.value(),
             sandbox_policy: turn_context.sandbox_policy.get().clone(),
             network: None,
@@ -8823,6 +8855,42 @@ mod tests {
         assert!(environment_update.contains("<denied>blocked.example.com</denied>"));
     }
 
+    #[tokio::test]
+    async fn build_settings_update_items_emits_environment_item_for_time_changes() {
+        let (session, previous_context) = make_session_and_context().await;
+        let previous_context = Arc::new(previous_context);
+        let mut current_context = previous_context
+            .with_model(
+                previous_context.model_info.slug.clone(),
+                &session.services.models_manager,
+            )
+            .await;
+        current_context.current_date = Some("2026-02-27".to_string());
+        current_context.timezone = Some("Europe/Berlin".to_string());
+
+        let reference_context_item = previous_context.to_turn_context_item();
+        let update_items = session.build_settings_update_items(
+            Some(&reference_context_item),
+            None,
+            &current_context,
+        );
+
+        let environment_update = update_items
+            .iter()
+            .find_map(|item| match item {
+                ResponseItem::Message { role, content, .. } if role == "user" => {
+                    let [ContentItem::InputText { text }] = content.as_slice() else {
+                        return None;
+                    };
+                    text.contains("<environment_context>").then_some(text)
+                }
+                _ => None,
+            })
+            .expect("environment update item should be emitted");
+        assert!(environment_update.contains("<current_date>2026-02-27</current_date>"));
+        assert!(environment_update.contains("<timezone>Europe/Berlin</timezone>"));
+    }
+
     #[tokio::test]
     async fn record_context_updates_and_set_reference_context_item_injects_full_context_when_baseline_missing()
      {