diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 2a2993dedb25d..ce44ce3261286 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -567,21 +567,6 @@ def _parsed_string_to_bounds(self, reso: Resolution, parsed: datetime):
         -------
         lower, upper: pd.Timestamp
         """
-        assert isinstance(reso, Resolution), (type(reso), reso)
-        valid_resos = {
-            "year",
-            "month",
-            "quarter",
-            "day",
-            "hour",
-            "minute",
-            "second",
-            "millisecond",
-            "microsecond",
-        }
-        if reso.attrname not in valid_resos:
-            raise KeyError
-
         grp = reso.freq_group
         per = Period(parsed, freq=grp.value)
         start, end = per.start_time, per.end_time
@@ -590,17 +575,17 @@ def _parsed_string_to_bounds(self, reso: Resolution, parsed: datetime):
         # If an incoming date string contained a UTC offset, need to localize
         # the parsed date to this offset first before aligning with the index's
         # timezone
+        start = start.tz_localize(parsed.tzinfo)
+        end = end.tz_localize(parsed.tzinfo)
+
         if parsed.tzinfo is not None:
             if self.tz is None:
                 raise ValueError(
                     "The index must be timezone aware when indexing "
                     "with a date string with a UTC offset"
                 )
-            start = start.tz_localize(parsed.tzinfo).tz_convert(self.tz)
-            end = end.tz_localize(parsed.tzinfo).tz_convert(self.tz)
-        elif self.tz is not None:
-            start = start.tz_localize(self.tz)
-            end = end.tz_localize(self.tz)
+        start = self._maybe_cast_for_get_loc(start)
+        end = self._maybe_cast_for_get_loc(end)
         return start, end
 
     def _can_partial_date_slice(self, reso: Resolution) -> bool:
diff --git a/pandas/core/indexes/period.py b/pandas/core/indexes/period.py
index 306695d99bead..ecaff162b4b74 100644
--- a/pandas/core/indexes/period.py
+++ b/pandas/core/indexes/period.py
@@ -503,16 +503,8 @@ def _parsed_string_to_bounds(self, reso: Resolution, parsed: datetime):
 
     def _can_partial_date_slice(self, reso: Resolution) -> bool:
         assert isinstance(reso, Resolution), (type(reso), reso)
-        grp = reso.freq_group
-        freqn = self.dtype.freq_group_code
-
-        if not grp.value < freqn:
-            # TODO: we used to also check for
-            #  reso in ["day", "hour", "minute", "second"]
-            #  why is that check not needed?
-            return False
-
-        return True
+        # e.g. test_getitem_setitem_periodindex
+        return reso > self.dtype.resolution
 
 
 def period_range(