core: type user and auth session timestamps

crates/crank-core/src/access.rs

@@ -1,4 +1,5 @@
 use serde::{Deserialize, Serialize};
+use time::OffsetDateTime;
 
 use crate::ids::{InvitationId, PlatformApiKeyId, UserId, WorkspaceId};
 
@@ -48,7 +49,8 @@ pub struct User {
     pub email: String,
     pub display_name: String,
     pub status: UserStatus,
-    pub created_at: String,
+    #[serde(with = "time::serde::rfc3339")]
+    pub created_at: OffsetDateTime,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
@@ -82,3 +84,44 @@ pub struct PlatformApiKey {
     pub created_at: String,
     pub last_used_at: Option<String>,
 }
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+    use time::{OffsetDateTime, format_description::well_known::Rfc3339};
+
+    use super::{User, UserStatus};
+    use crate::ids::UserId;
+
+    #[test]
+    fn user_serializes_created_at_as_rfc3339() {
+        let user = User {
+            id: UserId::new("user_01"),
+            email: "owner@example.com".to_owned(),
+            display_name: "Owner".to_owned(),
+            status: UserStatus::Active,
+            created_at: OffsetDateTime::parse("2026-03-25T12:00:00Z", &Rfc3339).unwrap(),
+        };
+
+        let value = serde_json::to_value(&user).unwrap();
+
+        assert_eq!(value["created_at"], json!("2026-03-25T12:00:00Z"));
+    }
+
+    #[test]
+    fn user_deserializes_created_at_from_rfc3339() {
+        let user: User = serde_json::from_value(json!({
+            "id": "user_01",
+            "email": "owner@example.com",
+            "display_name": "Owner",
+            "status": "active",
+            "created_at": "2026-03-25T12:00:00Z"
+        }))
+        .unwrap();
+
+        assert_eq!(
+            user.created_at,
+            OffsetDateTime::parse("2026-03-25T12:00:00Z", &Rfc3339).unwrap()
+        );
+    }
+}