Add performance for UTC timestamps

asv_bench/benchmarks/timestamp.py

@@ -87,7 +87,7 @@ def time_microsecond(self, tz, freq):
 
 
 class TimestampOps(object):
-    params = [None, 'US/Eastern']
+    params = [None, 'US/Eastern', 'UTC']
     param_names = ['tz']
 
     def setup(self, tz):

doc/source/whatsnew/v0.24.0.rst

@@ -1134,7 +1134,7 @@ Performance Improvements
 - Improved performance of :func:`IndexEngine.get_indexer_non_unique` for sorted, non-unique indexes (:issue:`9466`)
 - Improved performance of :func:`PeriodIndex.unique` (:issue:`23083`)
 - Improved performance of :func:`pd.concat` for `Series` objects (:issue:`23404`)
-- Improved performance of :meth:`DatetimeIndex.normalize` and :meth:`Timestamp.normalize` for timezone naive datetimes (:issue:`23634`)
+- Improved performance of :meth:`DatetimeIndex.normalize` and :meth:`Timestamp.normalize` for timezone naive or UTC datetimes (:issue:`23634`)
 
 
 .. _whatsnew_0240.docs:

pandas/_libs/tslibs/timestamps.pyx

@@ -1286,9 +1286,10 @@ class Timestamp(_Timestamp):
         Normalize Timestamp to midnight, preserving
         tz information.
         """
-        if self.tz is None:
+        if self.tz is None or is_utc(self.tz):
             DAY_NS = DAY_SECONDS * 1000000000
-            return Timestamp(self.value - (self.value % DAY_NS))
+            normalized_value = self.value - (self.value % DAY_NS)
+            return Timestamp(normalized_value).tz_localize(self.tz)
         normalized_value = normalize_i8_timestamps(
             np.array([self.value], dtype='i8'), tz=self.tz)[0]
         return Timestamp(normalized_value).tz_localize(self.tz)

pandas/_libs/tslibs/timezones.pxd

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-cdef bint is_utc(object tz)
+cpdef bint is_utc(object tz)
 cdef bint is_tzlocal(object tz)
 
 cdef bint treat_tz_as_pytz(object tz)

pandas/_libs/tslibs/timezones.pyx

@@ -27,7 +27,7 @@ cdef int64_t NPY_NAT = get_nat()
 
 # ----------------------------------------------------------------------
 
-cdef inline bint is_utc(object tz):
+cpdef inline bint is_utc(object tz):
     return tz is UTC or isinstance(tz, _dateutil_tzutc)
 
 

pandas/core/arrays/datetimes.py

@@ -853,7 +853,7 @@ def normalize(self):
                        '2014-08-01 00:00:00+05:30'],
                        dtype='datetime64[ns, Asia/Calcutta]', freq=None)
         """
-        if self.tz is None:
+        if self.tz is None or timezones.is_utc(self.tz):
             not_null = self.notnull()
             DAY_NS = ccalendar.DAY_SECONDS * 1000000000
             new_values = self.asi8.copy()