diff --git a/src/adsb/combine_chunks_to_csv.py b/src/adsb/combine_chunks_to_csv.py
index 113a45e..6153ede 100644
--- a/src/adsb/combine_chunks_to_csv.py
+++ b/src/adsb/combine_chunks_to_csv.py
@@ -84,8 +84,12 @@ def combine_compressed_chunks(compressed_dfs: list[pl.DataFrame]) -> pl.DataFram
     return combined
 
 
-def download_and_merge_base_release(compressed_df: pl.DataFrame) -> pl.DataFrame:
-    """Download base release and merge with new data."""
+def download_and_merge_base_release(compressed_df: pl.DataFrame) -> tuple[pl.DataFrame, str | None]:
+    """Download base release and merge with new data.
+    
+    Returns:
+        Tuple of (merged_df, earliest_date_str) where earliest_date_str is None if no base release was merged
+    """
     from src.get_latest_release import download_latest_aircraft_adsb_csv
     
     print("Downloading base ADS-B release...")
@@ -100,6 +104,19 @@ def download_and_merge_base_release(compressed_df: pl.DataFrame) -> pl.DataFrame
             base_df = pl.read_csv(base_path)
             print(f"Base release has {len(base_df)} records")
             
+            # Extract earliest date from base release
+            earliest_date = None
+            if 'time' in base_df.columns and len(base_df) > 0:
+                try:
+                    earliest_timestamp = base_df['time'].min()
+                    if earliest_timestamp:
+                        # Parse timestamp and extract date
+                        earliest_dt = datetime.fromisoformat(str(earliest_timestamp).replace('Z', '+00:00'))
+                        earliest_date = earliest_dt.strftime('%Y-%m-%d')
+                        print(f"Earliest date in base release: {earliest_date}")
+                except Exception as e:
+                    print(f"Could not extract earliest date from base release: {e}")
+            
             # Ensure columns match
             base_cols = set(base_df.columns)
             new_cols = set(compressed_df.columns)
@@ -126,15 +143,15 @@ def download_and_merge_base_release(compressed_df: pl.DataFrame) -> pl.DataFrame
             del base_df, combined
             gc.collect()
             
-            return deduplicated
+            return deduplicated, earliest_date
         else:
             print(f"No base release found at {base_path}, using only new data")
-            return compressed_df
+            return compressed_df, None
     except Exception as e:
         import traceback
         print(f"Failed to download base release: {e}")
         traceback.print_exc()
-        return compressed_df
+        return compressed_df, None
 
 
 def cleanup_chunks(output_id: str, chunks_dir: str):
@@ -221,8 +238,15 @@ def main():
     print(f"After combining: {get_resource_usage()}")
     
     # Merge with base release (unless skipped)
+    base_start_date = None
     if not args.skip_base:
-        combined = download_and_merge_base_release(combined)
+        combined, base_start_date = download_and_merge_base_release(combined)
+        
+        # Update filename if we merged with base release and got a start date
+        if base_start_date and not (args.start_date and args.end_date):
+            # Only update filename for daily mode when base was merged
+            output_filename = f"openairframes_adsb_{base_start_date}_{date_str}.csv.gz"
+            print(f"Updated filename to reflect date range: {output_filename}")
     
     # Convert list columns to strings for CSV compatibility
     for col in combined.columns: