fix interpreting int as second--> nano

doc/source/whatsnew/v0.24.0.rst

@@ -436,6 +436,7 @@ Backwards incompatible API changes
 - Incorrectly passing a :class:`DatetimeIndex` to :meth:`MultiIndex.from_tuples`, rather than a sequence of tuples, now raises a ``TypeError`` rather than a ``ValueError`` (:issue:`24024`)
 - :func:`pd.offsets.generate_range` argument ``time_rule`` has been removed; use ``offset`` instead (:issue:`24157`)
 - In 0.23.x, pandas would raise a ``ValueError`` on a merge of a numeric column (e.g. ``int`` dtyped column) and an ``object`` dtyped column (:issue:`9780`). We have re-enabled the ability to merge ``object`` and other dtypes; pandas will still raise on a merge between a numeric and an ``object`` dtyped column that is composed only of strings (:issue:`21681`)
+- Passing an integer to :method:`Series.fillna` on a ``Series`` with ``timedelta64[ns]`` dtype will now treat that integer as a number of nanoseconds instead of a number of seconds (:issue:`24694`)
 
 Percentage change on groupby
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

pandas/core/internals/blocks.py

@@ -2499,9 +2499,9 @@ def _can_hold_element(self, element):
     def fillna(self, value, **kwargs):
 
         # allow filling with integers to be
-        # interpreted as seconds
+        # interpreted as nanoseconds
         if is_integer(value) and not isinstance(value, np.timedelta64):
-            value = Timedelta(value, unit='s')
+            value = self._box_func(value)
         return super(TimeDeltaBlock, self).fillna(value, **kwargs)
 
     def _try_coerce_args(self, values, other):

pandas/tests/series/test_missing.py

@@ -17,7 +17,7 @@
 import pandas as pd
 from pandas import (
     Categorical, DataFrame, Index, IntervalIndex, MultiIndex, NaT, Series,
-    Timestamp, date_range, isna)
+    Timedelta, Timestamp, date_range, isna)
 from pandas.core.series import remove_na
 import pandas.util.testing as tm
 from pandas.util.testing import assert_frame_equal, assert_series_equal
@@ -70,9 +70,10 @@ def test_timedelta_fillna(self):
                            timedelta(days=1, seconds=9 * 3600 + 60 + 1)])
         assert_series_equal(result, expected)
 
-        # interprested as seconds
+        # interprested as nanoseconds
         result = td.fillna(1)
-        expected = Series([timedelta(seconds=1), timedelta(0), timedelta(1),
+        expected = Series([Timedelta(nanoseconds=1),
+                           timedelta(0), timedelta(1),
                            timedelta(days=1, seconds=9 * 3600 + 60 + 1)])
         assert_series_equal(result, expected)