BUG: to_datetime when called with a unit and coerce is buggy

doc/source/whatsnew/v0.18.1.txt

@@ -428,6 +428,30 @@ In addition to this error change, several others have been made as well:
 - ``pd.read_csv()`` no longer allows a combination of strings and integers for the ``usecols`` parameter (:issue:`12678`)
 
 
+.. _whatsnew_0181.api.to_datetime:
+
+``to_datetime`` error changes
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Bugs in ``pd.to_datetime()`` when passing a ``unit`` with convertible entries and ``errors='coerce'`` or non-convertible with ``errors='ignore'`` (:issue:`11758`)
+
+Previous behaviour:
+
+.. code-block:: python
+
+   In [27]: pd.to_datetime(1420043460, unit='s', errors='coerce')
+   Out[27]: NaT
+
+   In [28]: pd.to_datetime(11111111, unit='D', errors='ignore')
+   OverflowError: Python int too large to convert to C long
+
+New behaviour:
+
+.. ipython:: python
+
+   pd.to_datetime(1420043460, unit='s', errors='coerce')
+   pd.to_datetime(11111111, unit='D', errors='ignore')
+
 .. _whatsnew_0181.api.other:
 
 Other API changes
@@ -444,7 +468,6 @@ Other API changes
 - ``pd.concat(ignore_index=True)`` now uses ``RangeIndex`` as default (:issue:`12695`)
 - ``pd.merge()`` and ``DataFrame.join()`` will show a ``UserWarning`` when merging/joining a single- with a multi-leveled dataframe (:issue:`9455`, :issue:`12219`)
 
-
 .. _whatsnew_0181.deprecations:
 
 Deprecations
@@ -514,7 +537,6 @@ Bug Fixes
 - Bug in aligning a ``Series`` with a ``DataFrame`` (:issue:`13037`)
 
 
-
 - Bug in consistency of ``.name`` on ``.groupby(..).apply(..)`` cases (:issue:`12363`)
 
 

pandas/io/json.py

@@ -400,7 +400,7 @@ def _try_convert_to_date(self, data):
             try:
                 new_data = to_datetime(new_data, errors='raise',
                                        unit=date_unit)
-            except OverflowError:
+            except ValueError:
                 continue
             except:
                 break

pandas/tseries/tests/test_timeseries.py

@@ -4151,6 +4151,7 @@ def test_basics_nanos(self):
         self.assertEqual(stamp.nanosecond, 500)
 
     def test_unit(self):
+
         def check(val, unit=None, h=1, s=1, us=0):
             stamp = Timestamp(val, unit=unit)
             self.assertEqual(stamp.year, 2000)
@@ -4217,6 +4218,44 @@ def check(val, unit=None, h=1, s=1, us=0):
         result = Timestamp('NaT')
         self.assertIs(result, NaT)
 
+    def test_unit_errors(self):
+        # GH 11758
+        # test proper behavior with erros
+
+        with self.assertRaises(ValueError):
+            to_datetime([1], unit='D', format='%Y%m%d')
+
+        values = [11111111, 1, 1.0, tslib.iNaT, pd.NaT, np.nan,
+                  'NaT', '']
+        result = to_datetime(values, unit='D', errors='ignore')
+        expected = Index([11111111, Timestamp('1970-01-02'),
+                          Timestamp('1970-01-02'), pd.NaT,
+                          pd.NaT, pd.NaT, pd.NaT, pd.NaT],
+                         dtype=object)
+        tm.assert_index_equal(result, expected)
+
+        result = to_datetime(values, unit='D', errors='coerce')
+        expected = DatetimeIndex(['NaT', '1970-01-02', '1970-01-02',
+                                  'NaT', 'NaT', 'NaT', 'NaT', 'NaT'])
+        tm.assert_index_equal(result, expected)
+
+        with self.assertRaises(ValueError):
+            to_datetime(values, unit='D', errors='raise')
+
+        values = [1420043460000, tslib.iNaT, pd.NaT, np.nan, 'NaT']
+
+        result = to_datetime(values, errors='ignore', unit='s')
+        expected = Index([1420043460000, pd.NaT, pd.NaT,
+                          pd.NaT, pd.NaT], dtype=object)
+        tm.assert_index_equal(result, expected)
+
+        result = to_datetime(values, errors='coerce', unit='s')
+        expected = DatetimeIndex(['NaT', 'NaT', 'NaT', 'NaT', 'NaT'])
+        tm.assert_index_equal(result, expected)
+
+        with self.assertRaises(ValueError):
+            to_datetime(values, errors='raise', unit='s')
+
     def test_roundtrip(self):
 
         # test value to string and back conversions

pandas/tseries/tools.py

@@ -170,7 +170,7 @@ def _guess_datetime_format_for_array(arr, **kwargs):
                  mapping={True: 'coerce', False: 'raise'})
 def to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
                 utc=None, box=True, format=None, exact=True, coerce=None,
-                unit='ns', infer_datetime_format=False):
+                unit=None, infer_datetime_format=False):
     """
     Convert argument to datetime.
 
@@ -293,7 +293,7 @@ def to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
 
 def _to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
                  utc=None, box=True, format=None, exact=True,
-                 unit='ns', freq=None, infer_datetime_format=False):
+                 unit=None, freq=None, infer_datetime_format=False):
     """
     Same as to_datetime, but accept freq for
     DatetimeIndex internal construction
@@ -323,9 +323,17 @@ def _convert_listlike(arg, box, format, name=None):
                 arg = arg.tz_convert(None).tz_localize('UTC')
             return arg
 
-        elif format is None and com.is_integer_dtype(arg) and unit == 'ns':
-            result = arg.astype('datetime64[ns]')
+        elif unit is not None:
+            if format is not None:
+                raise ValueError("cannot specify both format and unit")
+            arg = getattr(arg, 'values', arg)
+            result = tslib.array_with_unit_to_datetime(arg, unit,
+                                                       errors=errors)
             if box:
+                if errors == 'ignore':
+                    from pandas import Index
+                    return Index(result, dtype=object)
+
                 return DatetimeIndex(result, tz='utc' if utc else None,
                                      name=name)
             return result
@@ -387,7 +395,6 @@ def _convert_listlike(arg, box, format, name=None):
                     dayfirst=dayfirst,
                     yearfirst=yearfirst,
                     freq=freq,
-                    unit=unit,
                     require_iso8601=require_iso8601
                 )