fix(prompt): make prompt timestamps timezone-explicit (#10209)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Douwe M Osinga <douwe@sidewalklabs.com>

crates/goose/src/agents/moim.rs

@@ -142,7 +142,7 @@ fn compose_moim(
     max_turns: u32,
     extension_parts: Vec<String>,
 ) -> String {
-    let timestamp = chrono::Local::now().format("%Y-%m-%d %H:%M:00");
+    let timestamp = chrono::Local::now().format("%Y-%m-%d %H:%M:00 %:z");
     let mut lines = vec![
         open_tag(TURN_CONTEXT_TAG),
         tag(CURRENT_TIME_TAG, &timestamp.to_string()),
@@ -437,5 +437,21 @@ mod tests {
                  match: the detector's shape checks are what discriminate the injected block"
             );
         }
+
+        #[test]
+        fn current_time_is_timezone_aware() {
+            let block = moim(None, None, 0, 0, vec![]);
+            let time_line = block
+                .lines()
+                .find(|line| line.starts_with(&format!("<{CURRENT_TIME_TAG}>")))
+                .expect("turn-context block should contain a current-time line");
+
+            let value = time_line
+                .trim_start_matches(&format!("<{CURRENT_TIME_TAG}>"))
+                .trim_end_matches(&format!("</{CURRENT_TIME_TAG}>"));
+
+            chrono::DateTime::parse_from_str(value, "%Y-%m-%d %H:%M:%S %:z")
+                .expect("current-time should include a numeric UTC offset");
+        }
     }
 }

crates/goose/src/agents/prompt_manager.rs

@@ -206,7 +206,7 @@ impl PromptManager {
             system_prompt_extras: IndexMap::new(),
             // Use the fixed current date time so that prompt cache can be used.
             // Filtering to an hour to balance user time accuracy and multi session prompt cache hits.
-            current_date_timestamp: Utc::now().format("%Y-%m-%d %H:00").to_string(),
+            current_date_timestamp: Utc::now().format("%Y-%m-%d %H:00 %:z").to_string(),
             subdirectory_hint_tracker: SubdirectoryHintTracker::new(),
         }
     }
@@ -216,7 +216,7 @@ impl PromptManager {
         PromptManager {
             system_prompt_override: None,
             system_prompt_extras: IndexMap::new(),
-            current_date_timestamp: dt.format("%Y-%m-%d %H:%M:%S").to_string(),
+            current_date_timestamp: dt.format("%Y-%m-%d %H:%M:%S %:z").to_string(),
             subdirectory_hint_tracker: SubdirectoryHintTracker::new(),
         }
     }
@@ -300,6 +300,17 @@ mod tests {
         assert!(result.contains("with hidden text"));
     }
 
+    #[test]
+    fn test_current_date_time_includes_timezone() {
+        let mut manager =
+            PromptManager::with_timestamp(DateTime::<Utc>::from_timestamp(0, 0).unwrap());
+        manager.set_system_prompt_override("It is currently {{current_date_time}}".to_string());
+
+        let result = manager.builder().build();
+
+        assert_eq!(result, "It is currently 1970-01-01 00:00:00 +00:00");
+    }
+
     #[test]
     fn test_build_system_prompt_sanitizes_extras() {
         let mut manager = PromptManager::new();