diff --git a/calc_gps_map_csv.py b/calc_gps_map_csv.py
index 6b91778..a62db8b 100755
--- a/calc_gps_map_csv.py
+++ b/calc_gps_map_csv.py
@@ -2,6 +2,7 @@
 import multiprocessing
 import os
 from argparse import ArgumentParser
+from datetime import datetime
 from math import ceil
 from time import sleep
 
@@ -53,6 +54,7 @@ if __name__ == "__main__":
     parser.add_argument("--save", default=None, help="Location to save pdf file.")
     parser.add_argument("--time_offset", default=0, type=int, help="Minutes added to GPS datetime.")
     parser.add_argument("--neg_offset", default=False, action="store_true", help="Subtract GPS time offset.")
+    parser.add_argument("--auto_offset", default=False, action="store_true", help="Calculate GPS time offset.")
     parser.add_argument(
         "-c",
         "--cores",
@@ -132,13 +134,24 @@ if __name__ == "__main__":
     # load dataframe an put it into geopandas
     df = pd.read_csv(args.gps_file)
     df["kmh"] = df["speed (knots)"].apply(lambda x: x * 1.852)
-    if args.time_offset > 0:
+    if not args.auto_offset and args.time_offset > 0:
         if args.neg_offset:
             df["datetime"] = pd.to_datetime(df["datetime"]) - pd.Timedelta(minutes=args.time_offset)
         else:
             df["datetime"] = pd.to_datetime(df["datetime"]) + pd.Timedelta(minutes=args.time_offset)
+    elif args.auto_offset:
+        gps_first = datetime.strptime(df["datetime"].iloc[0], "%Y-%m-%d %H:%M:%S.%f")
+        pcap_first = datetime.strptime(transmission_df["datetime"].iloc[0], "%Y-%m-%d %H:%M:%S.%f")
+        calc_offset = gps_first - pcap_first
+        if gps_first > pcap_first:
+            time_offset = gps_first - pcap_first
+            df["datetime"] = pd.to_datetime(df["datetime"]) - time_offset
+        else:
+            time_offset = pcap_first - gps_first
+            df["datetime"] = pd.to_datetime(df["datetime"]) + time_offset
     else:
         df["datetime"] = pd.to_datetime(df["datetime"])
+
     df = df.set_index("datetime")
     df.index = pd.to_datetime(df.index)