TST: add test for compatibility with numpy datetime64 (#45766)

* TST: add test for compatibility with numpy datetime64 #41617

* TST; add test for compatibility with numpy datetime64 (#41617)

* Fixed a fat finger error in one of the expected values

* Fixes from pre-commit [automated commit]

* changed numpy import

Author: ChrisAlbertsen

pandas/tests/tseries/frequencies/test_freq_code.py

@@ -1,3 +1,4 @@
+import numpy as np
 import pytest
 
 from pandas._libs.tslibs import (
@@ -78,3 +79,19 @@ def test_cat(args):
 
     with pytest.raises(ValueError, match=msg):
         to_offset(str(args[0]) + args[1])
+
+
+@pytest.mark.parametrize(
+    "freqstr,expected",
+    [
+        ("1H", "2021-01-01T09:00:00"),
+        ("1D", "2021-01-02T08:00:00"),
+        ("1W", "2021-01-03T08:00:00"),
+        ("1M", "2021-01-31T08:00:00"),
+        ("1Y", "2021-12-31T08:00:00"),
+    ],
+)
+def test_compatibility(freqstr, expected):
+    ts_np = np.datetime64("2021-01-01T08:00:00.00")
+    do = to_offset(freqstr)
+    assert ts_np + do == np.datetime64(expected)