ENH: Timestamp.__sub__(datetime) with non-nano

Author: jbrockmendel

pandas/_libs/tslibs/timestamps.pyx

@@ -413,9 +413,6 @@ cdef class _Timestamp(ABCTimestamp):
                 result._set_freq(self._freq)  # avoid warning in constructor
             return result
 
-        elif isinstance(self, _Timestamp) and self._reso != NPY_FR_ns:
-            raise NotImplementedError(self._reso)
-
         elif is_integer_object(other):
             raise integer_op_not_supported(self)
 
@@ -450,9 +447,6 @@ cdef class _Timestamp(ABCTimestamp):
             neg_other = -other
             return self + neg_other
 
-        elif isinstance(self, _Timestamp) and self._reso != NPY_FR_ns:
-            raise NotImplementedError(self._reso)
-
         elif is_array(other):
             if other.dtype.kind in ['i', 'u']:
                 raise integer_op_not_supported(self)
@@ -483,10 +477,18 @@ cdef class _Timestamp(ABCTimestamp):
                     "Cannot subtract tz-naive and tz-aware datetime-like objects."
                 )
 
+            # We allow silent casting to the lower resolution if and only
+            #  if it is lossless.
+            if self._reso < other._reso:
+                other = (<_Timestamp>other)._as_reso(self._reso, round_ok=False)
+            elif self._reso > other._reso:
+                self = (<_Timestamp>self)._as_reso(other._reso, round_ok=False)
+
             # scalar Timestamp/datetime - Timestamp/datetime -> yields a
             # Timedelta
             try:
-                return Timedelta(self.value - other.value)
+                res_value = self.value - other.value
+                return Timedelta._from_value_and_reso(res_value, self._reso)
             except (OverflowError, OutOfBoundsDatetime, OutOfBoundsTimedelta) as err:
                 if isinstance(other, _Timestamp):
                     if both_timestamps:
@@ -507,9 +509,6 @@ cdef class _Timestamp(ABCTimestamp):
         return NotImplemented
 
     def __rsub__(self, other):
-        if self._reso != NPY_FR_ns:
-            raise NotImplementedError(self._reso)
-
         if PyDateTime_Check(other):
             try:
                 return type(self)(other) - self

pandas/tests/scalar/timestamp/test_timestamp.py

@@ -22,6 +22,7 @@
 from pandas._libs.tslibs.timezones import (
     dateutil_gettz as gettz,
     get_timezone,
+    maybe_get_tz,
     tz_compare,
 )
 from pandas.errors import OutOfBoundsDatetime
@@ -712,6 +713,11 @@ def dt64(self, reso):
     def ts(self, dt64):
         return Timestamp._from_dt64(dt64)
 
+    @pytest.fixture
+    def ts_tz(self, ts, tz_aware_fixture):
+        tz = maybe_get_tz(tz_aware_fixture)
+        return Timestamp._from_value_and_reso(ts.value, ts._reso, tz)
+
     def test_non_nano_construction(self, dt64, ts, reso):
         assert ts.value == dt64.view("i8")
 
@@ -893,6 +899,60 @@ def test_addsub_timedeltalike_non_nano(self, dt64, ts, td):
         assert result._reso == ts._reso
         assert result == expected
 
+    @pytest.mark.xfail(reason="tz_localize not yet implemented for non-nano")
+    def test_addsub_offset(self, ts_tz):
+        # specifically non-Tick offset
+        off = offsets.YearBegin(1)
+        result = ts_tz + off
+
+        assert isinstance(result, Timestamp)
+        assert result._reso == ts_tz._reso
+        # If ts_tz is ever on the last day of the year, the year would be
+        #  incremented by one
+        assert result.year == ts_tz.year
+        assert result.day == 31
+        assert result.month == 12
+        assert tz_compare(result.tz, ts_tz.tz)
+
+    def test_sub_datetimelike_mismatched_reso(self, ts_tz):
+        # case with non-lossy rounding
+        ts = ts_tz
+
+        #  choose a unit for `other` that doesn't match ts_tz's
+        unit = {
+            NpyDatetimeUnit.NPY_FR_us.value: "ms",
+            NpyDatetimeUnit.NPY_FR_ms.value: "s",
+            NpyDatetimeUnit.NPY_FR_s.value: "us",
+        }[ts._reso]
+        other = ts._as_unit(unit)
+        assert other._reso != ts._reso
+
+        result = ts - other
+        assert isinstance(result, Timedelta)
+        assert result.value == 0
+        assert result._reso == min(ts._reso, other._reso)
+
+        result = other - ts
+        assert isinstance(result, Timedelta)
+        assert result.value == 0
+        assert result._reso == min(ts._reso, other._reso)
+
+        # TODO: clarify in message that add/sub is allowed only when lossless?
+        msg = "Cannot losslessly convert units"
+        if ts._reso < other._reso:
+            # Case where rounding is lossy
+            other2 = other + Timedelta._from_value_and_reso(1, other._reso)
+            with pytest.raises(ValueError, match=msg):
+                ts - other2
+            with pytest.raises(ValueError, match=msg):
+                other2 - ts
+        else:
+            ts2 = ts + Timedelta._from_value_and_reso(1, ts._reso)
+            with pytest.raises(ValueError, match=msg):
+                ts2 - other
+            with pytest.raises(ValueError, match=msg):
+                other - ts2
+
 
 class TestAsUnit:
     def test_as_unit(self):