BUG: Rounding error in Timestamp.floor() for dates far in the future and past (GH19206)

Author: cbertinato

doc/source/whatsnew/v0.23.0.txt

@@ -354,7 +354,6 @@ Documentation Changes
 Bug Fixes
 ~~~~~~~~~
 
-
 Conversion
 ^^^^^^^^^^
 
@@ -462,6 +461,7 @@ Numeric
 - Bug in :class:`DatetimeIndex`, :class:`TimedeltaIndex` addition and subtraction of zero-dimensional integer arrays gave incorrect results (:issue:`19012`)
 - Bug in :func:`Series.__add__` adding Series with dtype ``timedelta64[ns]`` to a timezone-aware ``DatetimeIndex`` incorrectly dropped timezone information (:issue:`13905`)
 - Bug in :func:`Timedelta.__floordiv__` and :func:`Timedelta.__rfloordiv__` dividing by many incompatible numpy objects was incorrectly allowed (:issue:`18846`)
+- Bug in :func:`Timestamp.floor` where time stamps far in the future and past were not rounded correctly (:issue:`19206`)
 -
 
 Categorical

pandas/_libs/tslibs/timestamps.pyx

@@ -600,17 +600,30 @@ class Timestamp(_Timestamp):
             value = self.tz_localize(None).value
         else:
             value = self.value
+
+        # GH 19206
+        # to deal with round-off when unit is large
+        if unit >= 1e9:
+            divisor = 10 ** int(np.log10(unit / 1e7))
+        else:
+            divisor = 1
+
         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'))
+                 (rounder((value % buff) * (1 / 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'))
+            r = (unit *
+                 rounder((value * divisor / float(unit)) / divisor)
+                 .astype('i8'))
         else:
-            r = (unit * rounder(value / float(unit)).astype('i8'))
+            r = (unit *
+                 rounder((value * divisor / float(unit)) / divisor)
+                 .astype('i8'))
+
         result = Timestamp(r, unit='ns')
         if self.tz is not None:
             result = result.tz_localize(self.tz)

pandas/tests/scalar/test_timestamp.py

@@ -695,6 +695,27 @@ def test_round(self):
         expected = Timestamp('20130101')
         assert result == expected
 
+        # GH 19206
+        dt = Timestamp('2117-01-01 00:00:45')
+        result = dt.floor('15s')
+        expected = Timestamp('2117-01-01 00:00:45')
+        assert result == expected
+
+        dt = Timestamp('2117-01-01 00:00:45.000000012')
+        result = dt.floor('10ns')
+        expected = Timestamp('2117-01-01 00:00:45.000000010')
+        assert result == expected
+
+        dt = Timestamp('1823-01-01 00:00:01')
+        result = dt.floor('1s')
+        expected = Timestamp('1823-01-01 00:00:01')
+        assert result == expected
+
+        dt = Timestamp('1823-01-01 00:00:01.000000012')
+        result = dt.floor('10ns')
+        expected = Timestamp('1823-01-01 00:00:01.000000010')
+        assert result == expected
+
         # ceil
         dt = Timestamp('20130101 09:10:11')
         result = dt.ceil('D')