FIX BUG: Timestamp __add__/__sub__ DateOffset with nanoseconds lost.

pandas/_libs/tslibs/offsets.pyx

@@ -180,7 +180,7 @@ def apply_wraps(func):
             result = result.normalize()
 
         # nanosecond may be deleted depending on offset process
-        if not self.normalize and nano != 0:
+        if not self.normalize and nano != 0 and not hasattr(self, "nanoseconds"):
             if result.nanosecond != nano:
                 if result.tz is not None:
                     # convert to UTC
@@ -326,7 +326,7 @@ cdef _determine_offset(kwds):
             # sub-daily offset - use timedelta (tz-aware)
             offset = timedelta(**kwds_no_nanos)
     else:
-        offset = timedelta(1)
+        offset = timedelta(0)
     return offset, use_relativedelta
 
 
@@ -1046,12 +1046,17 @@ cdef class RelativeDeltaOffset(BaseOffset):
                 # perform calculation in UTC
                 other = other.replace(tzinfo=None)
 
+            if hasattr(self, "nanoseconds"):
+                td_nano = Timedelta(nanoseconds=self.nanoseconds)
+            else:
+                td_nano = Timedelta(0)
+
             if self.n > 0:
                 for i in range(self.n):
-                    other = other + self._offset
+                    other = other + self._offset + td_nano
             else:
                 for i in range(-self.n):
-                    other = other - self._offset
+                    other = other - self._offset - td_nano
 
             if tzinfo is not None and self._use_relativedelta:
                 # bring tz back from UTC calculation

pandas/_libs/tslibs/timestamps.pyx

@@ -307,12 +307,11 @@ cdef class _Timestamp(ABCTimestamp):
         elif not isinstance(self, _Timestamp):
             # cython semantics, args have been switched and this is __radd__
             return other.__add__(self)
-
         return NotImplemented
 
     def __sub__(self, other):
 
-        if is_any_td_scalar(other) or is_integer_object(other):
+        if (is_any_td_scalar(other) or is_integer_object(other)):
             neg_other = -other
             return self + neg_other
 

pandas/tests/tseries/offsets/test_offsets.py

@@ -657,14 +657,6 @@ def test_rule_code(self):
                 assert alias == (_get_offset(alias) * 5).rule_code
 
 
-def test_dateoffset_misc():
-    oset = offsets.DateOffset(months=2, days=4)
-    # it works
-    oset.freqstr
-
-    assert not offsets.DateOffset(months=2) == 2
-
-
 def test_freq_offsets():
     off = BDay(1, offset=timedelta(0, 1800))
     assert off.freqstr == "B+30Min"
@@ -780,6 +772,41 @@ def test_tick_normalize_raises(tick_classes):
         cls(n=3, normalize=True)
 
 
+@pytest.mark.parametrize(
+    "cases",
+    [
+        ("nanoseconds", 1, Timestamp("1970-01-01 00:00:00.000000001")),
+        ("nanoseconds", 5, Timestamp("1970-01-01 00:00:00.000000005")),
+        ("nanoseconds", -1, Timestamp("1969-12-31 23:59:59.999999999")),
+        ("microseconds", 1, Timestamp("1970-01-01 00:00:00.000001")),
+        ("microseconds", -1, Timestamp("1969-12-31 23:59:59.999999")),
+        ("seconds", 1, Timestamp("1970-01-01 00:00:01")),
+        ("seconds", -1, Timestamp("1969-12-31 23:59:59")),
+        ("minutes", 1, Timestamp("1970-01-01 00:01:00")),
+        ("minutes", -1, Timestamp("1969-12-31 23:59:00")),
+        ("hours", 1, Timestamp("1970-01-01 01:00:00")),
+        ("hours", -1, Timestamp("1969-12-31 23:00:00")),
+        ("days", 1, Timestamp("1970-01-02 00:00:00")),
+        ("days", -1, Timestamp("1969-12-31 00:00:00")),
+        ("weeks", 1, Timestamp("1970-01-08 00:00:00")),
+        ("weeks", -1, Timestamp("1969-12-25 00:00:00")),
+        ("months", 1, Timestamp("1970-02-01 00:00:00")),
+        ("months", -1, Timestamp("1969-12-01 00:00:00")),
+        ("years", 1, Timestamp("1971-01-01 00:00:00")),
+        ("years", -1, Timestamp("1969-01-01 00:00:00")),
+    ],
+)
+def test_dateoffset_add_sub(cases):
+    time_unit, num, expected = cases
+    offset = DateOffset(**{time_unit: num})
+    ts = Timestamp(0) + offset
+    assert ts == expected
+    ts -= offset
+    assert ts == Timestamp(0)
+    ts = offset + Timestamp(0)
+    assert ts == expected
+
+
 @pytest.mark.parametrize(
     "attribute",
     [
@@ -795,3 +822,11 @@ def test_dateoffset_immutable(attribute):
     msg = "DateOffset objects are immutable"
     with pytest.raises(AttributeError, match=msg):
         setattr(offset, attribute, 5)
+
+
+def test_dateoffset_misc():
+    oset = offsets.DateOffset(months=2, days=4)
+    # it works
+    oset.freqstr
+
+    assert not offsets.DateOffset(months=2) == 2