diff --git a/pandas/tests/tools/test_to_datetime.py b/pandas/tests/tools/test_to_datetime.py
index 9da7951c199ca..7351f50aea8c1 100644
--- a/pandas/tests/tools/test_to_datetime.py
+++ b/pandas/tests/tools/test_to_datetime.py
@@ -2530,3 +2530,15 @@ def test_empty_string_datetime_coerce__unit():
     # verify that no exception is raised even when errors='raise' is set
     result = to_datetime([1, ""], unit="s", errors="raise")
     tm.assert_index_equal(expected, result)
+
+
+@pytest.mark.parametrize("cache", [True, False])
+def test_to_datetime_monotonic_increasing_index(cache):
+    # GH28238
+    cstart = start_caching_at
+    times = date_range(Timestamp("1980"), periods=cstart, freq="YS")
+    times = times.to_frame(index=False, name="DT").sample(n=cstart, random_state=1)
+    times.index = times.index.to_series().astype(float) / 1000
+    result = to_datetime(times.iloc[:, 0], cache=cache)
+    expected = times.iloc[:, 0]
+    tm.assert_series_equal(result, expected)