DEPR: make to_datetime('now') match Timestamp('now')

doc/source/whatsnew/v1.4.0.rst

@@ -603,6 +603,7 @@ Other Deprecations
 - Deprecated passing non boolean argument to sort in :func:`concat` (:issue:`41518`)
 - Deprecated passing arguments as positional for :func:`read_fwf` other than ``filepath_or_buffer`` (:issue:`41485`):
 - Deprecated passing ``skipna=None`` for :meth:`DataFrame.mad` and :meth:`Series.mad`, pass ``skipna=True`` instead (:issue:`44580`)
+- Deprecated the behavior of :func:`to_datetime` with the string "now" with ``utc=False``; in a future version this will match ``Timestamp("now")``, which in turn matches :meth:`Timestamp.now` (:issue:`18705`)
 - Deprecated :meth:`DateOffset.apply`, use ``offset + other`` instead (:issue:`44522`)
 - Deprecated parameter ``names`` in :meth:`Index.copy` (:issue:`44916`)
 - A deprecation warning is now shown for :meth:`DataFrame.to_latex` indicating the arguments signature may change and emulate more the arguments to :meth:`.Styler.to_latex` in future versions (:issue:`44411`)
@@ -616,7 +617,6 @@ Other Deprecations
 - Deprecated ``numeric_only=None`` in :meth:`DataFrame.rank`; in a future version ``numeric_only`` must be either ``True`` or ``False`` (the default) (:issue:`45036`)
 - Deprecated the behavior of :meth:`Timestamp.utcfromtimestamp`, in the future it will return a timezone-aware UTC :class:`Timestamp` (:issue:`22451`)
 - Deprecated :meth:`NaT.freq` (:issue:`45071`)
--
 
 .. ---------------------------------------------------------------------------
 

pandas/_libs/tslib.pyx

@@ -1,3 +1,5 @@
+import warnings
+
 import cython
 
 from cpython.datetime cimport (
@@ -516,7 +518,7 @@ cpdef array_to_datetime(
                     if string_to_dts_failed:
                         # An error at this point is a _parsing_ error
                         # specifically _not_ OutOfBoundsDatetime
-                        if _parse_today_now(val, &iresult[i]):
+                        if _parse_today_now(val, &iresult[i], utc):
                             continue
                         elif require_iso8601:
                             # if requiring iso8601 strings, skip trying
@@ -755,14 +757,23 @@ cdef _array_to_datetime_object(
     return oresult, None
 
 
-cdef inline bint _parse_today_now(str val, int64_t* iresult):
+cdef inline bint _parse_today_now(str val, int64_t* iresult, bint utc):
     # We delay this check for as long as possible
     # because it catches relatively rare cases
-    if val == 'now':
-        # Note: this is *not* the same as Timestamp('now')
+    if val == "now":
         iresult[0] = Timestamp.utcnow().value
+        if not utc:
+            # GH#18705 make sure to_datetime("now") matches Timestamp("now")
+            warnings.warn(
+                "The parsing of 'now' in pd.to_datetime without `utc=True` is "
+                "deprecated. In a future version, this will match Timestamp('now') "
+                "and Timestamp.now()",
+                FutureWarning,
+                stacklevel=1,
+            )
+
         return True
-    elif val == 'today':
+    elif val == "today":
         iresult[0] = Timestamp.today().value
         return True
     return False

pandas/tests/arithmetic/test_datetime64.py

@@ -1679,8 +1679,8 @@ def test_dt64_series_arith_overflow(self):
         tm.assert_series_equal(res, -expected)
 
     def test_datetimeindex_sub_timestamp_overflow(self):
-        dtimax = pd.to_datetime(["now", Timestamp.max])
-        dtimin = pd.to_datetime(["now", Timestamp.min])
+        dtimax = pd.to_datetime(["2021-12-28 17:19", Timestamp.max])
+        dtimin = pd.to_datetime(["2021-12-28 17:19", Timestamp.min])
 
         tsneg = Timestamp("1950-01-01")
         ts_neg_variants = [
@@ -1718,8 +1718,8 @@ def test_datetimeindex_sub_timestamp_overflow(self):
 
     def test_datetimeindex_sub_datetimeindex_overflow(self):
         # GH#22492, GH#22508
-        dtimax = pd.to_datetime(["now", Timestamp.max])
-        dtimin = pd.to_datetime(["now", Timestamp.min])
+        dtimax = pd.to_datetime(["2021-12-28 17:19", Timestamp.max])
+        dtimin = pd.to_datetime(["2021-12-28 17:19", Timestamp.min])
 
         ts_neg = pd.to_datetime(["1950-01-01", "1950-01-01"])
         ts_pos = pd.to_datetime(["1980-01-01", "1980-01-01"])

pandas/tests/tools/test_to_datetime.py

@@ -591,9 +591,15 @@ def test_to_datetime_unparseable_ignore(self):
     def test_to_datetime_now(self):
         # See GH#18666
         with tm.set_timezone("US/Eastern"):
-            npnow = np.datetime64("now").astype("datetime64[ns]")
-            pdnow = to_datetime("now")
-            pdnow2 = to_datetime(["now"])[0]
+            msg = "The parsing of 'now' in pd.to_datetime"
+            with tm.assert_produces_warning(
+                FutureWarning, match=msg, check_stacklevel=False
+            ):
+                # checking stacklevel is tricky because we go through cython code
+                # GH#18705
+                npnow = np.datetime64("now").astype("datetime64[ns]")
+                pdnow = to_datetime("now")
+                pdnow2 = to_datetime(["now"])[0]
 
             # These should all be equal with infinite perf; this gives
             # a generous margin of 10 seconds
@@ -632,7 +638,12 @@ def test_to_datetime_today(self, tz):
 
     @pytest.mark.parametrize("arg", ["now", "today"])
     def test_to_datetime_today_now_unicode_bytes(self, arg):
-        to_datetime([arg])
+        warn = FutureWarning if arg == "now" else None
+        msg = "The parsing of 'now' in pd.to_datetime"
+        with tm.assert_produces_warning(warn, match=msg, check_stacklevel=False):
+            # checking stacklevel is tricky because we go through cython code
+            # GH#18705
+            to_datetime([arg])
 
     @pytest.mark.parametrize(
         "dt", [np.datetime64("2000-01-01"), np.datetime64("2000-01-02")]