diff --git a/csharp/App/Backend/DeleteOldData/DeleteOldDataFromS3.cs b/csharp/App/Backend/DeleteOldData/DeleteOldDataFromS3.cs
index 6eb53179b..8327d58f2 100644
--- a/csharp/App/Backend/DeleteOldData/DeleteOldDataFromS3.cs
+++ b/csharp/App/Backend/DeleteOldData/DeleteOldDataFromS3.cs
@@ -54,7 +54,7 @@ public static class DeleteOldDataFromS3
                 var bucket = s3Region.Bucket(installation.BucketName());
 
                 Console.WriteLine($"[S3Cleanup] Processing {installation.Name} (bucket: {bucket.Name})");
-                var deleted = await DeleteObjectsBefore(bucket, cutoffKey);
+                var deleted = await DeleteObjectsBefore(bucket, cutoffTimestamp);
                 Console.WriteLine($"[S3Cleanup] {installation.Name}: deleted {deleted} objects");
             }
             catch (Exception ex)
@@ -95,8 +95,11 @@ public static class DeleteOldDataFromS3
 
                 await foreach (var obj in bucket.ListObjects())
                 {
-                    if (string.Compare(obj.Path, cutoffKey, StringComparison.Ordinal) >= 0)
-                        break;
+                    if (!TryGetChunkTimestamp(obj.Path, out var ts))
+                        continue; // skip aggregated DDMMYYYY.json and any non-chunk object
+
+                    if (ts >= cutoffTimestamp)
+                        break; // chunks are listed in ascending order; everything after is newer
 
                     hasOldData = true;
                     if (sampleKeys.Count < 5)
@@ -121,15 +124,33 @@ public static class DeleteOldDataFromS3
         return string.Join("\n", results);
     }
 
-    private static async Task<int> DeleteObjectsBefore(S3Bucket bucket, string cutoffKey)
+    // Raw 10-second data chunks are named "{unixSeconds}.json"/".csv" (10-digit timestamps).
+    // Device-generated aggregated files are named "DDMMYYYY.json" (max 8 digits -> < 1e9).
+    // Only timestamp chunks may be deleted; the threshold cleanly separates the two formats and
+    // avoids deleting aggregated daily files by lexicographic accident.
+    private const Int64 MinUnixTimestamp = 1_000_000_000;
+
+    private static Boolean TryGetChunkTimestamp(String key, out Int64 timestamp)
+    {
+        timestamp = 0;
+        var file = key.Substring(key.LastIndexOf('/') + 1);
+        var dot  = file.IndexOf('.');
+        var stem = dot >= 0 ? file.Substring(0, dot) : file;
+        return Int64.TryParse(stem, out timestamp) && timestamp >= MinUnixTimestamp;
+    }
+
+    private static async Task<int> DeleteObjectsBefore(S3Bucket bucket, long cutoffTimestamp)
     {
         var totalDeleted = 0;
         var keysToDelete = new List<string>();
 
         await foreach (var obj in bucket.ListObjects())
         {
-            if (string.Compare(obj.Path, cutoffKey, StringComparison.Ordinal) >= 0)
-                break;
+            if (!TryGetChunkTimestamp(obj.Path, out var ts))
+                continue; // skip aggregated DDMMYYYY.json and any non-chunk object
+
+            if (ts >= cutoffTimestamp)
+                break; // chunks are listed in ascending order; everything after is newer
 
             keysToDelete.Add(obj.Path);