Fix fast partition count calculations for daily partitions definition (…

…#22046)

Summary:
owen tried to warn me about this on
https://github.com/dagster-io/dagster/pull/21791/files, and in my hubris
I did not listen - the test coverage here was not as thorough as i
thought it was. Fix the fast calculations when you have a simple daily
partitions definition (the existing logic gets messed up by DST
boundaries when they are on opposite sides of the boundary).

Test Plan: BK, shadow graphql of an incorrect partitions count

## Summary & Motivation

## How I Tested These Changes

python_modules/dagster/dagster/_core/definitions/time_window_partitions.py

@@ -3,7 +3,7 @@
 import json
 import re
 from abc import abstractmethod, abstractproperty
-from datetime import datetime, timedelta
+from datetime import date, datetime, timedelta
 from enum import Enum
 from functools import cached_property
 from typing import (
@@ -383,7 +383,18 @@ def _get_fast_num_partitions(self, current_time: Optional[datetime] = None) -> O
             return None
 
         if self.is_basic_daily:
-            return (last_partition_window.start - first_partition_window.start).days + 1
+            return (
+                date(
+                    last_partition_window.start.year,
+                    last_partition_window.start.month,
+                    last_partition_window.start.day,
+                )
+                - date(
+                    first_partition_window.start.year,
+                    first_partition_window.start.month,
+                    first_partition_window.start.day,
+                )
+            ).days + 1
 
         fixed_minute_interval = get_fixed_minute_interval(self.cron_schedule)
         if fixed_minute_interval:

python_modules/dagster/dagster_tests/definitions_tests/test_time_window_partitions.py

@@ -1240,6 +1240,26 @@ def my_partitioned_config_2(_start, _end):
         partitions_def.get_partition_keys(current_time)
     )
 
+    @daily_partitioned_config(start_date="2020-01-01", timezone="US/Pacific")
+    def my_daily_dst_transition_partitioned_config(_start, _end):
+        return {}
+
+    partitions_def = cast(
+        TimeWindowPartitionsDefinition, my_daily_dst_transition_partitioned_config.partitions_def
+    )
+
+    current_time_post_transition = datetime.strptime("2024-05-22", "%Y-%m-%d")
+
+    assert partitions_def.get_num_partitions(current_time_post_transition) == len(
+        partitions_def.get_partition_keys(current_time_post_transition)
+    )
+
+    current_time_pre_transition = datetime.strptime("2024-02-01", "%Y-%m-%d")
+
+    assert partitions_def.get_num_partitions(current_time_pre_transition) == len(
+        partitions_def.get_partition_keys(current_time_pre_transition)
+    )
+
 
 def test_get_first_partition_window():
     assert DailyPartitionsDefinition(