fix some imports

test/dataset/test_process.py

@@ -0,0 +1,20 @@
+import pytest
+import pandas as pd
+
+from pts.dataset import ProcessStartField
+
+@pytest.mark.parametrize(
+    "freq, expected",
+    [
+        ("B", "2019-11-01"),
+        ("W", "2019-11-03"),
+        ("M", "2019-11-30"),
+        ("12M", "2019-11-30"),
+        ("A-DEC", "2019-12-31"),
+    ],
+)
+def test_process_start_field(freq, expected):
+    process = ProcessStartField.process
+    given = "2019-11-01 12:34:56"
+
+    assert process(given, freq) == pd.Timestamp(expected, freq)