diff --git a/src/mito2/src/compaction.rs b/src/mito2/src/compaction.rs
index b41f7319496f..424198439a88 100644
--- a/src/mito2/src/compaction.rs
+++ b/src/mito2/src/compaction.rs
@@ -12,6 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+mod buckets;
+mod picker;
+mod task;
+#[cfg(test)]
+mod test_util;
+mod twcs;
+mod window;
+
 use std::collections::HashMap;
 use std::sync::Arc;
 use std::time::{Duration, Instant};
@@ -54,14 +62,6 @@ use crate::sst::file_purger::FilePurgerRef;
 use crate::sst::version::LevelMeta;
 use crate::worker::WorkerListener;
 
-mod buckets;
-mod picker;
-mod task;
-#[cfg(test)]
-mod test_util;
-mod twcs;
-mod window;
-
 /// Region compaction request.
 pub struct CompactionRequest {
     pub(crate) engine_config: Arc<MitoConfig>,
diff --git a/src/mito2/src/compaction/window.rs b/src/mito2/src/compaction/window.rs
index ccd1203a6beb..ee4f7c181c9e 100644
--- a/src/mito2/src/compaction/window.rs
+++ b/src/mito2/src/compaction/window.rs
@@ -191,6 +191,7 @@ fn assign_files_to_time_windows<'a>(
         }
         let (start, end) = file.time_range();
         let bounds = file_time_bucket_span(
+            // safety: converting whatever timestamp to seconds will not overflow.
             start.convert_to(TimeUnit::Second).unwrap().value(),
             end.convert_to(TimeUnit::Second).unwrap().value(),
             bucket_sec,