Set date_format to ISO8601 to silence pandas 2.0 UserWarning on read_csv

Use `pd.read_csv(..., date_format="ISO8601")` to silence `UserWarning: Could not infer format, so each element will be parsed individually, falling back to `dateutil`. To ensure parsing is consistent and as-expected, please specify a format.`. Xref pandas-dev/pandas#50601

Author: weiji14

pygmt/src/x2sys_cross.py

@@ -47,7 +47,7 @@ def tempfile_from_dftrack(track, suffix):
             sep="\t",
             index=False,
             na_rep="NaN",  # write a NaN value explicitly instead of a blank string
-            date_format="%Y-%m-%dT%H:%M:%S.%fZ",
+            date_format="%Y-%m-%dT%H:%M:%S.%fZ",  # ISO8601 format
         )
         yield tmpfilename
     finally:
@@ -235,6 +235,7 @@ def x2sys_cross(tracks=None, outfile=None, **kwargs):
                     header=2,  # Column names are on 2nd row
                     comment=">",  # Skip the 3rd row with a ">"
                     parse_dates=[2, 3],  # Datetimes on 3rd and 4th column
+                    date_format="ISO8601",
                 )
                 # Remove the "# " from "# x" in the first column
                 table = table.rename(columns={table.columns[0]: table.columns[0][2:]})

pygmt/tests/test_x2sys_cross.py

@@ -112,7 +112,7 @@ def test_x2sys_cross_input_two_dataframes(mock_x2sys_home):
         for i in range(2):
             np.random.seed(seed=i)
             track = pd.DataFrame(data=np.random.rand(10, 3), columns=("x", "y", "z"))
-            track["time"] = pd.date_range(start=f"2020-{i}1-01", periods=10, freq="ms")
+            track["time"] = pd.date_range(start=f"2020-{i}1-01", periods=10, freq="min")
             tracks.append(track)
 
         output = x2sys_cross(tracks=tracks, tag=tag, coe="e")