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BUG: Fix dt64[non_nano] + some_offsets incorrectly rounding #62383

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BUG: Fix dt64[non_nano] + some_offsets incorrectly rounding #62383

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11 changes: 10 additions & 1 deletion pandas/core/arrays/datetimes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -817,7 +817,16 @@ def _add_offset(self, offset: BaseOffset) -> Self:
result = type(self)._from_sequence(res_values, dtype=self.dtype)

else:
result = type(self)._simple_new(res_values, dtype=res_values.dtype)
units = ["ns", "us", "ms", "s", "m", "h", "D"]
res_unit = self.unit
if hasattr(offset, "offset"):
offset_unit = Timedelta(offset.offset).unit
idx_self = units.index(self.unit)
idx_offset = units.index(offset_unit)
res_unit = units[min(idx_self, idx_offset)]
dtype = tz_to_dtype(self.tz, unit=res_unit)
result = type(self)._simple_new(res_values, dtype=dtype)

if offset.normalize:
result = result.normalize()
result._freq = None
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17 changes: 17 additions & 0 deletions pandas/tests/arrays/test_datetimes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -844,3 +844,20 @@ def test_factorize_sort_without_freq():
tda = dta - dta[0]
with pytest.raises(NotImplementedError, match=msg):
tda.factorize(sort=True)


def test_dt64_non_nano_offset_no_rounding():
# GH#56586
dti = pd.date_range("2016-01-01", periods=3, unit="s")
offset = pd.offsets.CustomBusinessDay(offset=pd.Timedelta("1ms"))
result = dti + offset

assert result.dtype == np.dtype("datetime64[ms]")
expected = pd.DatetimeIndex(
[
pd.Timestamp("2016-01-02 00:00:00.001"),
pd.Timestamp("2016-01-03 00:00:00.001"),
pd.Timestamp("2016-01-04 00:00:00.001"),
]
)
assert all(result == expected)
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