Adds support for negative time offset in gps timestamp.

calc_gps_map_csv.py

     parser.add_argument("-s", "--serial_file", required=True, help="Serial csv file.")
     parser.add_argument("-p", "--pcap_csv_folder", required=True, help="PCAP csv folder.")
     parser.add_argument("--save", default=None, help="Location to save pdf file.")
-    parser.add_argument("--time_offset", default=None, type=int, help="Minutes added to GPS datetime.")
+    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(
         "-c",
         "--cores",
     # 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 is not None:
-        if args.time_offset > 0:
-            df["datetime"] = pd.to_datetime(df["datetime"]) + pd.Timedelta(minutes=abs(args.time_offset))
+    if 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=abs(args.time_offset))
+            df["datetime"] = pd.to_datetime(df["datetime"]) + pd.Timedelta(minutes=args.time_offset)
     else:
         df["datetime"] = pd.to_datetime(df["datetime"])
     df = df.set_index("datetime")