Adds support for negative time offset in gps timestamp.

calc_gps_map_csv.py

     # 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:
-        df["datetime"] = pd.to_datetime(df["datetime"]) + pd.Timedelta(minutes=args.time_offset)
+    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))
+        else:
+            df["datetime"] = pd.to_datetime(df["datetime"]) - pd.Timedelta(minutes=abs(args.time_offset))
     else:
         df["datetime"] = pd.to_datetime(df["datetime"])
     df = df.set_index("datetime")