core: type observability timestamps

crates/crank-core/src/observability.rs

@@ -1,5 +1,6 @@
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
+use time::OffsetDateTime;
 
 use crate::{AgentId, OperationId, WorkspaceId};
 
@@ -63,7 +64,8 @@ pub struct InvocationLog {
     pub error_kind: Option<String>,
     pub request_preview: Value,
     pub response_preview: Value,
-    pub created_at: String,
+    #[serde(with = "time::serde::rfc3339")]
+    pub created_at: OffsetDateTime,
 }
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
@@ -79,3 +81,52 @@ pub struct UsageRollup {
     pub p95_ms: u64,
     pub p99_ms: u64,
 }
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+    use time::{OffsetDateTime, format_description::well_known::Rfc3339};
+
+    use super::{InvocationLevel, InvocationLog, InvocationSource, InvocationStatus, UsagePeriod};
+    use crate::{AgentId, OperationId, WorkspaceId, ids::InvocationLogId};
+
+    fn timestamp(value: &str) -> OffsetDateTime {
+        OffsetDateTime::parse(value, &Rfc3339).unwrap()
+    }
+
+    #[test]
+    fn invocation_log_serializes_timestamps_as_rfc3339() {
+        let log = InvocationLog {
+            id: InvocationLogId::new("log_01"),
+            workspace_id: WorkspaceId::new("ws_01"),
+            agent_id: Some(AgentId::new("agent_01")),
+            operation_id: OperationId::new("op_01"),
+            source: InvocationSource::AdminTestRun,
+            level: InvocationLevel::Info,
+            status: InvocationStatus::Ok,
+            tool_name: "create_lead".to_owned(),
+            message: "ok".to_owned(),
+            request_id: Some("req_01".to_owned()),
+            status_code: Some(200),
+            duration_ms: 123,
+            error_kind: None,
+            request_preview: json!({"input": "value"}),
+            response_preview: json!({"ok": true}),
+            created_at: timestamp("2026-04-19T12:34:56Z"),
+        };
+
+        let value = serde_json::to_value(&log).unwrap();
+
+        assert_eq!(value["created_at"], "2026-04-19T12:34:56Z");
+        assert_eq!(value["source"], "admin_test_run");
+        assert_eq!(value["level"], "info");
+        assert_eq!(value["status"], "ok");
+    }
+
+    #[test]
+    fn usage_period_serializes_compact_labels() {
+        let value = serde_json::to_value(UsagePeriod::Last7Days).unwrap();
+
+        assert_eq!(value, "7d");
+    }
+}