BUG: Fixes rounding error in Timestamp.floor() (#19240)

doc/source/whatsnew/v0.23.0.txt

@@ -620,6 +620,7 @@ Datetimelike
 - Bug in :func:`~DataFrame.pct_change` using ``periods`` and ``freq`` returned different length outputs (:issue:`7292`)
 - Bug in comparison of :class:`DatetimeIndex` against ``None`` or ``datetime.date`` objects raising ``TypeError`` for ``==`` and ``!=`` comparisons instead of all-``False`` and all-``True``, respectively (:issue:`19301`)
 - Bug in :class:`Timestamp` and :func:`to_datetime` where a string representing a barely out-of-bounds timestamp would be incorrectly rounded down instead of raising ``OutOfBoundsDatetime`` (:issue:`19382`)
+- Bug in :func:`Timestamp.floor` :func:`DatetimeIndex.floor` where time stamps far in the future and past were not rounded correctly (:issue:`19206`)
 -
 
 Timezones

pandas/_libs/tslibs/timestamps.pyx

@@ -58,6 +58,46 @@ cdef inline object create_timestamp_from_ts(int64_t value,
     return ts_base
 
 
+def round_ns(values, rounder, freq):
+    """
+    Applies rounding function at given frequency
+
+    Parameters
+    ----------
+    values : int, :obj:`ndarray`
+    rounder : function
+    freq : str, obj
+
+    Returns
+    -------
+    int or :obj:`ndarray`
+    """
+    from pandas.tseries.frequencies import to_offset
+    unit = to_offset(freq).nanos
+    if unit < 1000:
+        # for nano rounding, work with the last 6 digits separately
+        # due to float precision
+        buff = 1000000
+        r = (buff * (values // buff) + unit *
+             (rounder((values % buff) * (1 / float(unit)))).astype('i8'))
+    else:
+        if unit % 1000 != 0:
+            msg = 'Precision will be lost using frequency: {}'
+            warnings.warn(msg.format(freq))
+
+        # GH19206
+        # to deal with round-off when unit is large
+        if unit >= 1e9:
+            divisor = 10 ** int(np.log10(unit / 1e7))
+        else:
+            divisor = 10
+
+        r = (unit * rounder((values * (divisor / float(unit))) / divisor)
+             .astype('i8'))
+
+    return r
+
+
 # This is PITA. Because we inherit from datetime, which has very specific
 # construction requirements, we need to do object instantiation in python
 # (see Timestamp class above). This will serve as a C extension type that
@@ -581,28 +621,12 @@ class Timestamp(_Timestamp):
         return create_timestamp_from_ts(ts.value, ts.dts, ts.tzinfo, freq)
 
     def _round(self, freq, rounder):
-
-        cdef:
-            int64_t unit, r, value, buff = 1000000
-            object result
-
-        from pandas.tseries.frequencies import to_offset
-        unit = to_offset(freq).nanos
         if self.tz is not None:
             value = self.tz_localize(None).value
         else:
             value = self.value
-        if unit < 1000 and unit % 1000 != 0:
-            # for nano rounding, work with the last 6 digits separately
-            # due to float precision
-            r = (buff * (value // buff) + unit *
-                 (rounder((value % buff) / float(unit))).astype('i8'))
-        elif unit >= 1000 and unit % 1000 != 0:
-            msg = 'Precision will be lost using frequency: {}'
-            warnings.warn(msg.format(freq))
-            r = (unit * rounder(value / float(unit)).astype('i8'))
-        else:
-            r = (unit * rounder(value / float(unit)).astype('i8'))
+
+        r = round_ns(value, rounder, freq)
         result = Timestamp(r, unit='ns')
         if self.tz is not None:
             result = result.tz_localize(self.tz)

pandas/core/indexes/datetimelike.py

@@ -36,6 +36,7 @@
 from pandas._libs import lib, iNaT, NaT
 from pandas._libs.tslibs.period import Period
 from pandas._libs.tslibs.timedeltas import delta_to_nanoseconds
+from pandas._libs.tslibs.timestamps import round_ns
 
 from pandas.core.indexes.base import Index, _index_shared_docs
 from pandas.util._decorators import Appender, cache_readonly
@@ -90,23 +91,9 @@ class TimelikeOps(object):
         """)
 
     def _round(self, freq, rounder):
-
-        from pandas.tseries.frequencies import to_offset
-        unit = to_offset(freq).nanos
         # round the local times
         values = _ensure_datetimelike_to_i8(self)
-        if unit < 1000 and unit % 1000 != 0:
-            # for nano rounding, work with the last 6 digits separately
-            # due to float precision
-            buff = 1000000
-            result = (buff * (values // buff) + unit *
-                      (rounder((values % buff) / float(unit))).astype('i8'))
-        elif unit >= 1000 and unit % 1000 != 0:
-            msg = 'Precision will be lost using frequency: {}'
-            warnings.warn(msg.format(freq))
-            result = (unit * rounder(values / float(unit)).astype('i8'))
-        else:
-            result = (unit * rounder(values / float(unit)).astype('i8'))
+        result = round_ns(values, rounder, freq)
         result = self._maybe_mask_results(result, fill_value=NaT)
 
         attribs = self._get_attributes_dict()

pandas/tests/indexes/datetimes/test_scalar_compat.py

@@ -126,6 +126,27 @@ def test_round(self, tz):
             ts = '2016-10-17 12:00:00.001501031'
             DatetimeIndex([ts]).round('1010ns')
 
+    @pytest.mark.parametrize('test_input, rounder, freq, expected', [
+        (['2117-01-01 00:00:45'], 'floor', '15s', ['2117-01-01 00:00:45']),
+        (['2117-01-01 00:00:45'], 'ceil', '15s', ['2117-01-01 00:00:45']),
+        (['2117-01-01 00:00:45.000000012'], 'floor', '10ns',
+         ['2117-01-01 00:00:45.000000010']),
+        (['1823-01-01 00:00:01.000000012'], 'ceil', '10ns',
+         ['1823-01-01 00:00:01.000000020']),
+        (['1823-01-01 00:00:01'], 'floor', '1s', ['1823-01-01 00:00:01']),
+        (['1823-01-01 00:00:01'], 'ceil', '1s', ['1823-01-01 00:00:01']),
+        (('NaT', '1823-01-01 00:00:01'), 'floor', '1s',
+         ('NaT', '1823-01-01 00:00:01')),
+        (('NaT', '1823-01-01 00:00:01'), 'ceil', '1s',
+         ('NaT', '1823-01-01 00:00:01'))
+    ])
+    def test_ceil_floor_edge(self, tz, test_input, rounder, freq, expected):
+        dt = DatetimeIndex(list(test_input))
+        func = getattr(dt, rounder)
+        result = func(freq)
+        expected = DatetimeIndex(list(expected))
+        assert expected.equals(result)
+
     # ----------------------------------------------------------------
     # DatetimeIndex.normalize
 

pandas/tests/scalar/timestamp/test_unary_ops.py

@@ -10,7 +10,7 @@
 
 from pandas.compat import PY3
 from pandas._libs.tslibs.frequencies import _INVALID_FREQ_ERROR
-from pandas import Timestamp
+from pandas import Timestamp, NaT
 
 
 class TestTimestampUnaryOps(object):
@@ -93,6 +93,29 @@ def test_round_frequencies(self, freq, expected):
         result = stamp.round(freq=freq)
         assert result == expected
 
+    @pytest.mark.parametrize('test_input, rounder, freq, expected', [
+        ('2117-01-01 00:00:45', 'floor', '15s', '2117-01-01 00:00:45'),
+        ('2117-01-01 00:00:45', 'ceil', '15s', '2117-01-01 00:00:45'),
+        ('2117-01-01 00:00:45.000000012', 'floor', '10ns',
+         '2117-01-01 00:00:45.000000010'),
+        ('1823-01-01 00:00:01.000000012', 'ceil', '10ns',
+         '1823-01-01 00:00:01.000000020'),
+        ('1823-01-01 00:00:01', 'floor', '1s', '1823-01-01 00:00:01'),
+        ('1823-01-01 00:00:01', 'ceil', '1s', '1823-01-01 00:00:01'),
+        ('NaT', 'floor', '1s', 'NaT'),
+        ('NaT', 'ceil', '1s', 'NaT')
+    ])
+    def test_ceil_floor_edge(self, test_input, rounder, freq, expected):
+        dt = Timestamp(test_input)
+        func = getattr(dt, rounder)
+        result = func(freq)
+
+        if dt is NaT:
+            assert result is NaT
+        else:
+            expected = Timestamp(expected)
+            assert result == expected
+
     def test_ceil(self):
         dt = Timestamp('20130101 09:10:11')
         result = dt.ceil('D')